Yeah, so the conclusion they seem to have is it's mostly in-engine.
While I would like a PC launch same day and date, I can see Rockstar trying to get this out as quickly as they can, and not developing for a 3rd ecosystem likely allows them to get the other 2 out quicker. I guess they could just focus on consoles (PS5 is generally the targetted platform for performance as market leader and adjustments are made around that). I guess Rockstar could half ass it like a ton of other studios have their PC ports recently, and then just patch it over time. Hopefully, the PC release is in pristine condition when it launches, unlike a decent amount of next-gen games for PC this year (For example, Jedi: Survivor and Hogwarts Legacy both games which I waited to buy on sale because of notable problems at launch).