His decision to turn it into a 6 part saga was at least the moment he titled ESB “Episode V” in the crawl.
He used EP V not because he wanted to make 6 part saga at that time but because he wanted to make 9 part saga.
it’s a six part saga that is meant to flow together.
Well I do not know how it was meant to be but the reality is that at the moment PT does not flow well with OT.
It’s not some subjective idea to be argued over whether or not Lucas wants I - VI to be considered one long story, it is a fact that Lucas wants I - VI to be considered one long story. You don’t have to agree, but that’s not the point.
It is completely irrelevant what Lucas thinks or wants. The reality is that plot-wise PT and OT are two very separate stories. They are two trilogies in the same universe, focusing on two different things.