I still question if you can actually see a difference between RF14 and RF20 on a good 1080p source. The Handbrake documentation even mentions that you probably shouldn't use RF lower than ~19 for DVD sources and that higher quality sources should use higher RF values. I'm not trying to tell you what to do, it's your computer/videos do you what you want, but maybe pick a 30 second clip or something and try a blind test of different RF values where you don't know which is which and see if you can really spot the RF14 video. But again, it's up to you.
Yes, you could start with a 5600x and move to a 5900x when it's more available. It will be way faster than your desktop CPU now. If you really don't care about encode times, then you might want to start using x265 as well, it's a more modern encoder and will produce smaller files at the same quality settings (but take longer).