I actually have experience porting games and engines to consoles. If it runs on a development PC (likely Windows), they have the build system and platform layer implemented, which is the hardest part. Porting the content is also an important step, but really only for consoles, which usually have limited memory and power.

Typically the only problem with "PC ports" today is when the game wasn't designed around mouse/keyboard, or when the devs didn't make an effort to optimize it on consumer specs (although nowadays console architecture isn't too different from PCs so there are more optimizations that work across platforms). Another potential problem is when the game gets a lot of last minute hacks to fix bugs in order to ship on a console and those hacks don't survive a platform transition, then the publisher just tells them to ship as is since there's no certification process on PC. Basically, the problems are almost always logistical/business decisions due to a lazy/cheap publisher.

None of that is going to apply to this game. Rockstar has always intended to ship and fully support PC from the beginning. They had the technology, the talent, the incentive, and the time to do it. The most realistic explanation (IMO) for the PC delay is that they're trying to double-dip.