After some thought, I'd say that one of these three options would do:
1) New dedicated trim sensitivity curve or bar
2) Dinamic input (like on the TF-51D for rudder trim, I believe): trimming input increases with the time the key is pressed (but starting from a value lower than it is now)
3) Additional trim keybindings with half the actual sensitivity (for more precise trimming)
Of the three, I believe, the last one would be the easiest to implement. :music_whistling: