A few things in this post are not accurate. The 5070 is supported by the nVidia Linux drivers since version 570.124.04. There’s also absolutely nothing wrong with using Ubuntu. They have a PPA for the newer nVidia drivers, which work fine with the current LTS release.
OP doesn’t say what kernel they’re running, or what version of Ubuntu. That would be useful information to have.
I think that’s generally right, if you just want to play games. I own a MiSTer and really like it; it’s convenient, it’s quick, and it’s really accurate. Where the single-core FPGA systems like the Spectrum Next and C64 Ultimate really come into their own is in things like physical peripherals and the tactile experience of the case and using the keyboard and so on. If that’s not important to you, then a MiSTer is going to be a better buy.