This is a very interesting question and i would like to write about more about it. If it is too much off-topic, i will move the discussion to another thread. Don’t want to distract too much from the also very interesting discussion about Proto-FInno-Ugrians here.
The first appearance of Indo-Iranians in history is around the middle of the 2nd millennium B.C. in the Hurrian-speaking state of Mittani, so it can be excluded that PII formed around or after this date. Also PII forming before 3000 B.C can be excluded because only around 3000 B.C the admixture event with GAC-like groups took place and Proto-CWC diverged into several major subcultures like Fatyanovo, Single Grave,Baltic CWC,.. So Proto-Indo-Iranian logically must form inbetween 1500-3000 B.C. Considering that Indo-Iranians shared a number of exclusive innovations seperating it from the other IE. Languages (see Quote) and need the same kind of Steppe_MLBA source rich in both basal R1a-Z93+ and derived R1a-Z94+ there must be a significant time of PII unity or at least very close contact. One interesting linguistic exception i recently only became aware of thanks to Pegasus is the absense of the Ruki law in Nuristani languages, what really points to an early divergence of Nuristani languages and being probably spoken on the most eastern area of the Proto-Indo-Iranian zone (Abashevo). Ruki law seems to effect Balto-Slavic, Armenian and maybe Albanian, what points to a feature spreading inbetween the Balkan, Ukraine and Volga-Ural region around 2000-2500 B.C (Catacomb-Late Middle Dnieper-Abashevo contacts?)
Based on this it is on one side unlikely that PII was already fully formed in early CWC, Middle Dnjepr or early Fatyanovo around 2800-3000 B.C (diverging from late PIEs probably already started there) and on the otherside that it only formed around 2000 B.C or even later. PII based on Vedic and Avestan literature/hymns seem to share a common Steppe cultural package with horse sacrificies/horse cults, horse-drawn chariots, sophisticated metallurgy, mobile agro-pastoralist economies,.. Especially chariots are so central for early Indo-Iranian mythology (most popular image and attribute of gods in early Rig Veda, that it is at the first view hard to imagine chariots or some kind of prototype being absent among PIIs. Early Iranics and Indo-Aryans also share the same term for chariot ( Vedic rátha , Avestan. raθa), which ultimately is derived from PIE *Hrót-h₂-os, from *Hret- (roll). From the same root many other IE terms for wheel are derived (Lithuanian ratas, Proto-Germanic raţą, Proto-Celtic rotos,..). Based on this it is very likely that the Proto-Indo-Iranian term was at first used for wheels and only later for horse-drawn chariots. So i would be careful about equating the first appereance of horse-drawn chariots with the first appereance of Proto-Indo-Iranians, because there are at least 500 if not 1000 years between the early appereance of Indo-Iranians with chariots in the Near East and South Asia and the first Proto-Indo-Iranians of East Europe. We can definetly say that chariots played a central role among already formed Iranics and Indo-Aryans and already late Proto-Indo-Iranians but i am not sure if the first PII communities already had horse-drawn chariots similar to later Sintashta and Steppe_MLBA. So far most date the earliest appereance of chariots to Sintashta around the 18-21th century, but there seems to be evidence fot the use of chariots or at least some kind of protypes in earlier Abashevo and Catacomb.(1) the merger of IE. *a, e, o and *ā, ē, ō into Indo-Ir. *a and *ā respectively (also in the diphthongs), (2) the development of IE. *ə into Indo-Ir. *i, (3) the change of IE. *s after *i, u, r, k into Indo-Ir. *š (Ir. *š, OInd. ṣ), (4) the gen. plur. ending *-nām in the vocalic stem classes, etc. In addition there are important correspondences in the vocabulary, especially in the field of religion and mythology, including morphological elements, such as suffixes and stem-formations, and phraseology. (See Chr. Bartholomae, “Vorgeschichte der iranischen Sprachen,” in Geiger and Kuhn, Grundr. Ir. Phil. I/1, 1895-1901, pp. 1-151. A. Erhart, Struktura indoíránských jazykűʷ [The structure of the Indo-Ir. languages], Brno, 1980.)
https://iranicaonline.org/articles/aryans
Much of archaelogy and literature is here still outdated and focused with Catacomb, Yamnaya or Poltavka being Proto-Indo-Iranian and Fatyanovo and even Abashevo being Baltic or “Western Indo-European“, what we thanks to genetic evidence can surely exclude now. Unfornuately this caused rearchers untill recently to not look deeper at the connections between Fatyanovo-Blanovo, Abashevo and Steppe_MLBA, This reminds me on the situation with Steppe Bell Beakers, which were untill recently mostly not associated with CWC and obvious links to Corded Ware were often ignored. But researchers were netherless very right about the archaeological and cultural links between Sintashta/Steppe_MLBA/early historical Indo-Iranians and Steppe culures of the Catacomb-Poltavka type. Even when genetic inflow fron Steppe_EBA seems to be rather limited at least in the earliest phases and before 2000 B.C there was a huge cultural impact for sure. Cultures like Potapovka and Sintashta seem to be basically Abashevo tribes imposing themselves on earlier Poltavka regions , so we find earlier Poltavka pottery there and older graves are reused (Potapovka). Much of the classical steppe cultural package of Steppe_MLBA like horse sacrificies, possibly chariots, sophisticated metallurgy and even pottery is either derived from Catacomb/Poltavka-type of cultures or was strongly influenced by them.The classification of cheek-pieces and the establishment of their evolution permits us to establish the origin of the disc-shaped cheek-pieces and their chronology. The most archaic disc-shaped cheek-piece was amorphous and undecorated of Type I and derived from contexts of the Catacomb-Multi-roller Ware and Abashevo cultures from the Ukraine to the Urals. This permits us to attribute the first controlling of chariots with cheek-pieces to tribes of the Abashevo and Multi-roller Ware cultures (KMK=Kul’tura Mnogovalikovoy Keramiki).
Sintashta being Proto-Indo-Iranian is also not likely in my eyes because despite many samples from several different sites much of it shows some very bottlenecked set of Y-DNA and i would expect Proto-Indo-Iranians to show more diverse basal R1a-Z93+ and Z94+ clades. Also it is too late, because than we just had several centuries between formed Mitanni Indo-Aryans distinct from even Vedic Aryans and Proto-Indo-Iranians, what would be really fast in my amateurish eyes. Rather Sintashta seems to be a Abashevo-derived Post-PII culture already closer to Iranics. But definetly an Indo-Iranian culture and of course important for understanding Proto-Indo-Iranians. So a slightly earlier culture ancestral to Sintashta is the most logical candidate for Proto-Indo-Iranians.
This ancestral culture is very likely Abashevo, which had prototypes/technologies for chariots, similar potttery, socketed spearheads, same type of cattle like Sintashta, anthropological /genetical similarities (especially GAC-like admix), militaristic orientation and overlapping sites. Also it seems to share recent drift with Sintashta based on Ukraine_MBA I6561 and Poltavka_O (likely belonging to Potapovka culture) sharing drift with Sintashta. Both samples were probably misdated and are related to Abashevo ultimately. Abashevo in turn shows close links to Fatyanovo-Balanovo and especially eastern Balanovo based on anthropological /genetical similarities, socketed spearheads (found earliest in Fatyanovo), similar pottery, carrying R1a-Z93, similar axes, overlapping sites/periods.
So i would date the earliest split of Proto-Indo-Iranians to around 2500 B.C (with Proto-Nuristani maybe diverging a bit earlier but staying closer to Proto-Indo-Aryan) in disintegrating eastern Fatyanovo-Balanovo and early Abashevo around the Middle Volga with close contacts between Proto-Iranics, Proto-Indo-Aryans, Proto-Nuristani and other extinct branches in Abashevo for the next 500 years before the migration into Central Asia and South Eurasia really started. Seemingly the intense inter-tribal warfare in Abashevo is a sign for the zone fracturing into different and distinct clans/dialects/tribal identies. Indo-Aryans likely departed slightly earlier from Abashevo into Andronovo than the Sintashta-Iranic wave and actually i have found sources mentioning Sintashta burning down older other Abashevo sites close to/in Andronovo. So there likely were earlier Abashevo migrations forming earliest Andronovo, which likely are either mostly overlooked, misinterpreted as coming from Sintashta or not deeper researched/found yet.
The origin and chronological correlation of early Andronovo sites are based on stratigraphy. On the Kuysak settlement ceramics with Pit-grave/Poltavka and Abashevo features were recovered from the earliest levels of the ditch. It indicates that these cultures participated in the formation of the Sintashta type of site (Smirnov and Kuz’mina 1976; 1977; Gening 1977). The early settlement built by the Pit-grave/Poltavka and Abashevo tribes was burnt, then it was rebuilt in the Sintashta period only to be fired again. All of this reflects the extremely tense atmosphere in inter-ethnic relations (Malyutina and Zdanovich 1995: 104- 105).