I’ve built my own arduino based trim control using a 10-turn potentiometer and the Trim Axis setting in MSFS. Super smooth and works well…until you change the trim in some other way (e.g. autopilot), then the next time you touch the trim wheel (or if you don’t code it right and there’s some jitter in the pot) then the trim will jump to the current trim wheel setting and not the small adjustment you though you were making.
So, using an up/down control approach is probably the better decision. Having a motorized pot as the trim wheel would be technically possible, but overly complex.