question

Fully agree.

Completely agree, having even just a basic implementation of the key mechanic of a THS (The combined Stab and Elev forces) available to the developer in the FM is essential. I'm having exactly that issue right now with poor trim authority at high AOA on an airliner.

I actually disagree with the initial point - the trim is not like a GA aircraft and is actually more like what you are requesting, but it has a crippling failure in that implimentation as well:

The trim on a GA aircraft is simply a tab which relieves the pressure at the stick, as you say. No matter how much trim is used, the elevator effect is bound by the deflection limits of the elevator itself and the trim purely creates a point within that at which there is minimal pressure on the stick. If you pull the stick fully aft in a situation where you can do that and maintain level flight, then adjusting the trim will alter the feel at the stick, but the aircraft will continue to maintain level flight. If you do this in the sim, then you will find that you gain extra pitch authority and are able to raise the nose further with the trim when the stick is already fully aft. I have just flown an entire circuit in the JF Warrior with the stick held fully aft and the trim alone used to adjust pitch, this should not be possible.

Taking the above, it appears that the trim is actually closer in effect to what you are asking for - THS. However, the problem then lies in the fact that the flight model does not allow you to define anything with regards to trim other than the angular limits. Therefore, the game is making its own mind up about the size of the trim tab and I would surmise that it has an area of 'a standard trim tab'. If you were able to define that area freely (i.e. the area of the 'trim tab' being the tailplane area), you would probably find that your airliners would behave correctly in this regard already.

This then creates a bug which is the opposite of what you have asked - trim does NOT work correctly on GA aircraft. This is not unique to MSFS, it is a legacy problem brought over from FSX. There literally is almost no point in having trim in a GA aircraft in the sim at this point in time, as explained earlier, the limits should be the up and down limits of the elevator.

Suggestions which would make the trim useable across the board as intended in real life:

1) A 'switch' in the .cfg for Standard/THS

2) Standard trim is coded so that it can only work within the defined elevator limits.

3) THS trim allows for the area of the trim 'tab' to be defined, no upper limit.

Hello.

This is in our roadmap but not short term.

Regards,
Sylvain