Freohr

This Mobo tier list is good for checking how good they are: That mobo is in tier A an is perfectly fine for your usecase, maybe even a bit overkill

R5 3600 is an excellent midrange cpu and right now game performance is mostly dominated by the graphics card. If you saved the $500+ from getting a 3600 instead of the 3950x, you could get a 2080 Ti, which is probably the best consumer gpu, not counting the RTX titan. That would be the best choice.

np

GPUs are used more in doing the calculations to run games so while your CPU is older, changing it wont see as much of a difference as getting a better GPU. A 2060 would boost fps more than an i5-5690 even though they are both 1 generation apart.

Gpu 100%. You'll see more performance gain.