I found the video bit underwhelming. Some oddities that stuck out were Etruria and Samnites (!) offering military alliance to Romans at the start and the feudatory vassal swarm.
In the first case there should be some relationship maluses to represent that the game doesn't begin from a blank state, but in middle of an interbellum in ongoing rivalry that is about to reach it's terminal stage (which historically was Rome and it's subjects vs. almost every other central Italian power, which Rome won decisively with some good maneuvering). Etruria and Samnites should both be hostile to Rome, while polities like Lucania that were threatened by Rome's rivals should be the ones seeking alliance. Quite likely that this is just WIP, of course. The latter, the vassal swarm, would get tiresome if this is supposed to be a long term solution to Socii and the like, and it's ahistorical for later (at least 2nd Punic War onwards) dates anyway when the Socii were integrated in the Roman military system under Roman commanders (with even their muster decreed from Rome with formula togatorum).
There are no later start dates. For 300 BC the socii as vassals which give the lord a manpower bonus makes sense. And we know from other streams and the press review event that we can diploannex vassals which would basically what you describe.