Instead of adapting your setup to accommodate your customary riding style (i.e. shortening your cranks), I suggest you keep the 138mm cranks on for a while and try to adapt your riding style to the trials setup. If this is completely new to you, if you’re struggling, that is a good thing. The twitch-iness of the setup will quicken your reflexes. Practice riding up impossibly steep hills with the 138mm cranks. Practice riding up a curb without unweighting (warning: even at high tire pressure you can hit bottom). Practice one footed still stands. Some new mounts may get easier with the longer cranks. Try to make the longer cranks the “right” cranks for the things you’re practicing. Otherwise you will be, to some extent, trying to emulate the larger wheel technique on the smaller wheel. Just my two cents…
I recently got a Athmos as well and swapped to 125s that I had. The seat is not very good compared to my Nimbus Unis. I haven’t put too much time on it yet but the 125s feel good and make it easier to do tight circle without fear of pedalstrike or making huge circles with my legs as i try to pedal quickly.
For reference I currently have
125 on the Athmos (may go back to 138 once I get the strength for bigger hopping tricks)
125 on my Freestyle 20"
138 on my Road 29" with Brake
150 on my 24" Nimbus Oracle MUNI with Brake
I must have worded that in a confusing way. It’s not that I found your contribution lacking, on the contrary I think it’s a good addition to the discussion of crank lengths here.
But I randomly remembered that in that thread I linked (it’s indeed a different thread to this one), a question was asked that you would now be in a great position to answer, since you now have experience on very similar setups with the only difference being a 19" tire vs. a 20" wheel. So all I wanted is for you to see that question, so that if you thought you have anything to add to that, you could.