Risa

For the reward of instantaneous travel, I’m sure the Federation could muddle its way through amending a 100 year old law.

Not really. The Federation, humanity most of all, are aggressively against genetic engineering. On top of it the genetic engineering requires one of those specific species of tardigrade. None were found despite Starfleet having a call put out to go looking for it. Only one was found and that was on accident.

The rest of the points don’t seem all that different than the complications we see our heroes regularly encounter exploring the galaxy.

Navigation in the other Trek shows isn't difficult. It's pathetically simple provided you're not going through some weird distortion or nebula that messes with a bunch of shit. Warp also doesn't destroy all known sentient life in the galaxy. Whether the Discovery keeps using it or not is irrelevant. At the time that the Spore Drive was known, it was not feasible to make another attempt at a spore drive. They did not have any of the originating scientists, they did not have required materials, and they were prohibited by their own law.

Janeway has no reason to be miffed given that she sat on an infinite speed drive herself, which had no downside that the doctor wouldn’t have been able to cure after it took them home.

Yep. That always made zero fucking sense to me. It's proven you can be un-salamandered and they have an inorganic being on board who wouldn't be affected. Why the hell don't they just Warp 10 back to the Alpha Quadrant? Or put everyone in stasis while they Warp 10 over? They've done it before. Janeway doesn't get to complain about a spore drive that would have required her to rebuild the nacelles from the ground up when she was sitting on a way home with a solution and didn't bother.