Welcome to the forum! I have not messed with the CVT belt yet, so I don't have an answer for that. From what I've seen, I don't think it is particularly difficult, but until I do it myself, I can't say for sure.
I don't have "sand mountains", but I do have sand/dirt/clay hills on my property that the 825i handles quite easily. I don't know if this answers your question, but if I start with a little speed, it keeps it right up. Stopping and then starting can tax it for just a bit, but once moving, I can accelerate pretty briskly. For an idea of the terrain/soil composition, I can stay in 2WD once moving, but starting up a slope requires 4WD. Also, I don't have really long runs before reaching level spots. It's no match for a fast ATV, but it's quite capable.
For snorkels, none from "aftermarket guys" yet, but I have seen reasonably nice looking "home built" solutions. I haven't tried it yet, but I will probably do something with mine in the near future. The CVT intake vent (on the driver's side) is also a little low and I will raise that as well. (I'm picky so it has to look "production" or I'd have thrown something together already.
) That still leaves the possibility of water going in the CVT exhaust, but I will see if that is an issue for me once the other mods are in place.
As for the gas tank, I agree it is quite small, and this is my biggest knock against the machine. I only have run on my property (35 acre ranch/farm) and around the neighborhood so far, so I haven't taxed it yet. I don't know of any solutions for this at this point.
- Steve in CO