As far as I know, the only royals (khattiyas) who called themselves Saka, Sākya or Śākyas were the Achaemenids of Persia (the ruling dynasty of Persia during the lifetime of the Buddha) - the word 'Achaemenid is a Greek form of the Old-Persian word Haxāmanišyaʰ (which would have been pronounced in Sanskrit (a sister language having the same Indo-Iranian origin as Old-Persian and Avestan) as Sakāmanuṣyāḥ or Sakhā-manuṣyāḥ (“the Sakā peoples”) - which is close enough to be considered a variant name of the hypothetical royal dynasty of the Buddha as per some of the EBTs.
A neighbouring related dynasty of the Sākyas is mentioned in Pali by the name Koliyas - perhaps this refers to (Sanskrit name) Kauravya (the dominant Kuru Kingdom of northern India whose kings were called by the patronymic Kauravya).
The Achaemenids ruled most of what is now Pakistan, although a large part of Pakistan is located within the Indian Subcontinent and their people were ethnically Indo-Aryan.
So the Buddha may have considered himself as belonging to Achaemenid ruled areas of India (primarily north-western India) including and upto the north-western reaches of the Ganges river.
So the Sākya-Koliya split may have been an Iranic-Indic split (coeval Sanskrit texts are usually silent about Achaemenid influences or rule during this period).
Beyond this I fail to see any historical reality or evidence of any Sākyas or Koliyas anywhere.
The Buddha himself in the Pali EBTs is called not just by his gotra (brahminical clan) patronymic Gotama (Gautama in Sanskrit) but also as Āṅgirasa (a bigger brahminical clan/gotra to which all Gautama brahmins were a part). He doesnt reject being called Āṅgirasa, which means he considered it to be a valid surname for himself. No royal (khattiya / kṣatriya) would inherit parent gotras (like Āṅgirasa for Gautama) along with their own gotra. He seems to however be at ease with his knowledge of Brahmanical sacrifices, Vedic culture, pre-Buddhist Vedic literature and brahmanical groups and clans. There is even a report in the Ambaṭṭha sutta in the DN where he is depicted as humbling the ego of someone from the ambaṭṭha clan by belittling the Ambaṭṭha’s brahmanical pedigree in plainly racist terms. So this buddha knows way too much about brahmins for him to be a kṣatriya, it appears to me.
So it seems the Buddha was a Brahmin (whose identity is for unknown reasons changed to that of a Kṣatriya).
There is no reference in any source to any political or military training he underwent in his youth which would have been mandatory for a kṣatriya. No reports of any wars or battles he participated in. No news of he or his father communicating with any other royals/kings of other kingdoms.
So much for the Buddha’s human identity.
Regarding early-Sāṅkhya being very possibly a form of early-Buddhism - it would explain why the founder of Sāṅkhya being exalted in Hindu texts as being the sage par excellence, while the Buddha of early Buddhism is conspicuous by his ubiquitous absence of all mention, even criticism, in Brahmanical texts of the era.
No I mean, most of the EBTs are full of enumerated lists (not just in the AN/EA). Noble eightfold path, four noble truths, twelve links of interdependent origination, ṣaḷ (six) āyatanas, three marks of existence. There are hundreds of enumerations like these in the EBTs.
Such extensive sets of enumerations are not found to such an extent in any other old-Indian philosophy as far as I am aware of.
In Sanskrit saṅkhyā means number, and sāṅkhya means a system of enumeration. As a name of a philosophy, it refers to one that was prominently using enumerations as an integral part of the philosophy, which fits early buddhism and the EBTs so well. These are my own conclusions though.