I got almost all of my wishes granted with RP2350

Would the pio now support sane Ethernet using rmii for example?

I'm assuming you've looked at the pico-rmii-ethernet library? If so, I feel your pain - I've been fixing issues, and am about halfway done. (This is for the DECstation2040 project, available on github). Look for a release in late aug/early sep. (Maybe with actual lance code? Dmitry??) The RP2350 will make RMII slightly easier - the endless DMA allows elimination of the DMA reload channel(s).

I looked at it and dismissed it as too hacky for production. I don't remember the real reason why. I would have to look through my notes. The main question is whether the RP2350 will change that. As in it actually possible to do bug free without weird hacks.

Agreed. I re-wrote the Rx PIO routine to do a proper interrupt at EOF, and added a DMA driven ring buffer, which eliminated a lot of the hackiness...