If I were to swap my cranks often (these are ISIS ones), would they loosen over time? I don’t want to repeat the amazing feat of having my crank fall out mid-ride but it doesn’t seem very possible to ride offroad with short cranks.
I’m riding a 29" with 125s. is it just a matter of getting used to or is it just not possible or realistic to ride offroad with that configuration?
I can ride singletrack and non-technical muni with a 29er with 125s. You could get used to the configuration, or if it is ISIS you could get those dual drilled KH moment cranks (but they are expensive).
As for the impact on ISIS hub/cranks from changing them a lot…I am not sure about that. I don’t have much experience with ISIS, but with my square taper super wide coker hub I have swapped cranks quite a lot and everything is still fine with the cranks and hub.
Get 89 mm cranks, the first time I tried to mount on that it was hard, but I tried it again and got it. Then I found it so easy to ride. You barely have to work at all and you can go alot faster. I think 89 is a good size because there is just enough power to spin the wheel, but short enough to cruise on a 29er.
I’ve had a blast riding “easy” trails with 125s on my Coker. That said, these were very non-techincal trails. If you have steep or technical stuff you want to be able to ride over, you’ll probably be happier with 150mm or longer. I use 160 on my 29" for MUni, but then again I haven’t used my 29" for MUni all year. At this point it may get mostly ignored in favor of the Coker (with 150s)…
I can’t speak about wear & tear from changing out ISIS cranks, but I know there is some wear & tear involved with square taper. However, most of that wear is to the threads where the crank puller goes. Alloy cranks do end up going on further and further over time, and may eventually have to be replaced if they bottom out and can no longer be tightened sufficiently. This has happened to me a few times, but I also have some 150mm IDOL alloy cranks (from when the Miyata 24" came with 150s) that are at least 25 years old, that have been on and off many unicycles over the years and they’re still doing fine.
got a source for 89mm ISIS cranks?
Swapping ISIS on and off, done properly, should not cause any damage. As with all cranks they will probably need retightening after the first ride, possibly several times. Invest in a high quality ISIS crank puller (a square taper one will ruin your hub thread).
90mm Koxx “ISIS” cranks. Shortest true ISIS cranks are the 100mm Qu-ax ones. However there have been rumors of qu-ax coming out with 89/90mm cranks, too.
My experience seems to indicate that the speed and smoothness benefits of 125s on my 29er diminish when a trail has plenty of drops. As long as the trail is relatively rollable though, it’s all good as long as I am fit enough to climb. Also, a brake is essential for long steep downhills.
Lately I’ve been doing a lot of mixed road and offroad rides, and stopping to change cranks is lame. 
If I were planning to only ride offroad, I’d probably go with cranks in the 137-152 mm range.
A little update to this post.
I’ve been trying to cycle fields these days to try out how 125s and 170s feel with the resistance. The most difficult aspect to 170s is mounting. Riding seems ok (theoretically) as long as you’ve good leg strength but still, over distance, I’d go for 170s. The only thing i hate about riding with long cranks is the amount of leg lift.
fluxus fwiw, is it possible for you to try 150’s too? I’ve been alternating between the 150 and 125 holes on the 29 (dual hole Moment cranks). I like both lengths, and it’s surprising how different the uni feels and behaves with a difference of 25mm. 150’s are great for the dirt riding and hills around here, the 125’s good for cruising, on flatter roads mainly, though my overall confidence is lower with the shorter cranks - a practice thing I guess!
I don’t have much long term interest in swapping cranks regularly tho, so it could for me as I improve something like 137’s would be a suitable all-round length crank on the 29 for the type of riding/terrain I enjoy.