Define "near future".
Destiny 2 (and Cyberpunk etc.) should be GPU-bound, especially after a CPU upgrade so I don't think you'll get 150 fps.
Rendering is basically infinitely parallelizable so it will use all the cores.
It's about the mobo and especially the VRMs, not the chipset.
Overclock does matter but 10% are 10%. He's not going to push a 2600 to 5 GHz so any 8 core CPU will win. The 2600 is neither here nor there. For single threaded/games he'd want the 3600 and if less ST is fine because the games are GPU-bound anyway he might as well go for more cheaper cores like clearance sales or used 1700(X)/1800X/2700(X) or even a 1900X/1920X if he wants to get really weird.
You don't really need the extra cores. Throwing out the cooler and mobo doesn't make sense either. A 3600(X) isn't really going to be faster than an overclocked 9600K(F) and a 3600(X) + mobo + cooler sure as hell aren't going to be cheaper than just the 9600K either.
An aftermarket 5700 XT would be a minor upgrade but again where are you going with this? NVidia finally caved in and supports Adaptive Sync now no matter how much they are trying to rebrand and hide it so what's stopping you from using it?