Posted by Jayne on the official forums - good news:

Thank you! Our team has identified four bugs with elevator trim and are prioritizing for fixes and release in future updates. They include:

• Elevator Trim is super sensitive when changed using the mouse wheel
• Elevator trim cannot be properly adjusted on aircraft that have electric trim switches (a regression)
• Full range of elevator trim cannot be achieved with Elevator Trim Axis (-100% to 100%) command
• Warning appears when user tries to map an axis to the Elevator Trim Axis (0 to 100%) command

They have been able to get the specific repros needed thanks to your help. Will keep you updated on progress when I can!

Thanks all.

[BUG LOGGED] Elevator trim doesn't work as pre-SU5 and it's impossible to fly - Bugs & Issues / Aircraft Systems - Microsoft Flight Simulator Forums

Good news