-PUBG is a terrible game to base a PC's performance off of, due to the lazy ass programming done by the developers.
Cpu, memory, and gpu optimization on that game is poor - the game doesn't run much better even on high end rigs, because the game can't properly use the extra resources.
-No 2 games are made the same. Some will run smoothly, others won't.[Stating 'other fps games is too broad!]
-It's certainly playable on a 9600k. But the cpu's integrated graphics aren't 1080p playable; 720p is the highest option.
-Z370 is not compatible with 9th gen Intel out of the box. A bios update must be done with an 8th gen cpu installed for it to work. Z390 is fine from the package.
-The only 'motherboard bottleneck'(if that's even a term) is a power limitation from pairing a 9900K/KF/KS with any non-Z390 motherboard - and a couple of those can't handle it either... well, still better than what Z370 does.
-8GBs of ram? That's the minimum now. Some games use more, some don't. Don't even consider getting a single stick kit, it hurts cpu performance. Dual channel > single channel.