Probably better asking about the quality in the XBox forums. 4K HDR games on my PS4 Pro look amazing so next gen will do as well.
AS for 4K @ 120fps, no chance with this TV but as you say 4K @ 60fps will be fine. The thing is to have AAA games running at 4K @120fps the graphics would have to be downgraded a lot (see Dirt 5 example) so 60fps is just fine and a lot of games I expect will only be 30fps with RT or under 4K, say 1440p @ 60fps.
I personally don't think there's much to worry about at all. If I was buying a TV today I would go for the LG CX just because it does support HDMI 2.1 but I'm not really going to be missing out much with my GZ