Trying to get back in on folding but probably on a more limited basis than previous. I would say you are likely better off selling the 3000 series cards based on prices I have seen. The going rate for the 3080 TI and 3090 is higher than 4070 supers. The 4070 super is higher performant and less power so more efficient.

I agree that X99s are great for folding if trying to do so on a budget. I am working out getting my 2080ti back in the game but unfortunately sold off most other cards last year since I had been laid off. Had been considering selling them anyway since a 4090 was doing the same PPD at less power than my entire setup(2x 3080, 2x 2080ti).

The WRX90 boards look like they might be interesting in about 5-6 years. Most of them have 5 x16 5.0 slots at full speed. The rarity of them might make the prices not come down like X299 though.