Hey, that's cool rep!
I've played or dabbled in most of it, so here's my thoughts:
Ondine is really hard BUT very approachable. What I mean is that, as long as you can play the first page without any tension, you won't hurt yourself with the piece. Just DO NOT play it if you can't play the first page on repeat for 10 minutes. It can damage your arms if you aren't relaxed.
Mazeppa is nasty. It was really frustrating to practice and honestly not that musically satisfying for a LONG time. So unless it comes easily to you I'd avoid it. Also very injury-inducing, though not so bad as Feux Follets.
Un Sospiro is easy.
The Chopin Ballades aren't bad except for the coda of number 4. So if you want to do number 4, see if the thirds at the end are manageable, and if they are then you are good to go. Number 1 is also hard as someone else said, the second and third aren't bad at all.
Rach 2 is pretty hard but honestly not as bad as people say EXCEPT for the third movement, in my humble opinion. The first theme of that movement and the development were just evil. However, this is also a piece that probably won't cause any injuries so I'd do it before Mazeppa. And the 2nd movement is mostly pretty easy so if you want to do that, go for it.
Appassionata I have only ever sightread, but I'd say that it's another case of difficult but not at all injury-producing. Overall one of the more approachable, it seems to me.
So overall I'd say that un sospiro is by far the easiest, and Appassionata and the 2nd and 3rd ballades aren't too much harder. Rach 2 1st and 2nd movement are also fine.
The rest is legitimately quite difficult, so approach it with caution, but if you can do it without worrying at all about your health, I'd say go for anything except for Mazeppa.
Oh and the second movement of Gaspard is frustrating but really good for colour development, and Scarbo takes years to play accurately and is very hard. But maybe not as taxing as Mazeppa from an endurance perspective.