The encoder must be mappable as an Axis, adding this because I have a “rotary encoder” wheel built into the logitech AP panel, which does not solve this issue at all as it only sends a button press up or down when scrolled up or down. Moving it fast just holds the button down. So yeah you’re right but hopefully OP know’s what you mean by this. Do we know what off the shelf hardware meets this criteria? Maybe good recommendation if OP can’t get it with buttons.
Not sure OP has the $ to drop on a peripheral to solve this.
I have the TCA Sidestick (T.16000M based) and using the built in Elevator Trim Up & Elevator Trim Down keybinds mapped to the hatswitch ontop of the joystick works fine to trim out most planes. I really don’t have any issues with the buttons and use them more than my trim wheel, then use the wheel to dial it in.
Sure you might not get a 0.0000001 level precision with buttons alone, but you can always move the throttle a tiny bit to compensate for that if you’re stuck in a slight climb or a slight descent.
Do expect to trim to level flight. Lots of people out there without rotary encoders achieving level flight with throttle & default keybinds for trim. I have flown the Blackbird PC-6 at 11,000FT over terrain for a full 2 hour flight without any autopilot at all. I didn’t deviate more than 200ft altitude. I did have to retrim a few times but was straight for at least 15 minutes at a time, it was very enjoyable.
OP, keep trying! I promise you will get it with practice 
and its very satisfying when you do! Another piece of advice is to do this over open water. The terrain affects turbulence significantly in this sim and if you’re going over constantly changing terrain, it can make trimming harder.
It would be worth learning in a less complicated plane though, as if the plane has rudder & aileron trim as well, these trim settings can inadvertently cause elevator trim to be more difficult. For example with a stick, if your aileron trim is not set for wings level, then you will be holding some slight left/right stick and its near impossible to do so without affecting the pitch of the plane as well. Once all are trimmed happy days though. You can leave that stick alone for some time
** oh wow sorry to necro this post just saw the date. Hope my posts are useful to someone.