Im probabally not the person you shouldnt be taking advice from, but Ill give it my best shot!
I have a 24" myself which I use for all of the same purposes as you. My cranks are 150mm, and Im not a beginner either; My 24" is actually my third uni. My first Uni had 110mm cranks but they were on a 16", so they’re kind of like 150’s on a 24"
I use it for everything, but your right about the length preventing you from doing certian things, and shorter ones would surely make you go faster.
Im thinking of buying another Uni, with 125mm cranks, but a 20" wheel (I really like trials) which I think would be much better. So, go with the 125’s.
But… like I said, you probabally shouldnt take advice from me… wait for some more replies before you make your decision!!
My 24 has a tyre with a 24 inch diameter (as opposed to one of those big fat tyres which make a 24 ‘effectively’ a 26).
I have ridden it OFF ROAD on cranks as follows: 150,125,110,102,89mm.
A 24 with 110s or 102s is quite a rapid machine. I once did about 15 miles in a ride on the 24 with 102s, and felt safe on the road with traffic (country lanes only). I once did some fairly serious (by my standards) cross country/MUni on the 24 on 110s.
You should have no problem on 125s, and the uni will be noticeably faster and smoother than with 150s.
It’s not a life changing decision. Basic cranks cost about the same as pizza for two with a beer each.