Dev here, great questions! I can answer some of these, hope this is helpful:
We have two modes for FPS: "consistent" which targets 30 fps in in both docked and handheld, as well as "responsive" which targets 40fps on compatible displays when docked (requires a 120hz TV) and ~40fps with VRR when handheld. We'd love to hit 60 but the game is very demanding performance wise since it's co-op and there are so many effects that occur in later stages of the run.
The graphics quality should feel pretty similar overall, very similar to 'normal' on PC with some minor adjustments.
There should be cross-play between Switch 2 and PC, Xbox X|S, and PS5, starting at launch. There is not cross-progression or shared save.
We also added mouse control for Switch 2, have seen a good number of players enjoy those for shooting feel.
One other thing to know is the 20% off launch discount for Switch 2 runs until March 30 - so if you're not sure about whether to get it, waiting a few days to see reviews etc. can't hurt.
All the best!