It sounds like the problem is you're relying on dampeners, I've mentioned in description that they won't work at all for this.
For example you only have one thruster pointing up with thrust reverser on it and closed, it won't make the ship hover at all, you have to manually get in a cockpit and press down to engate the thruster which then gets its thrust reversed and you go up.

The math in my code is pretty simple, I grab the thruster's current force and multiply it by 1.75 and apply it in the other direction, that 1 cancels the thrust's original force and the remaining 0.75 is the actual force.

Fixing this to work for dampeners/input is quite complicated, would be simpler to make a new one that has the reversers be thrusters themselves.

I would think that, but manually inputting the key doesn't give me any different results; only thrust override does.

Well post your blueprint or world so I can take a look at what's going on :}

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2325352192

Well I turned off all other thruster types, flipped it upside down and it can't hover with just those thrusts regardless of manual input or thrust override.
Added 1 big atmos with reverser and it can barely accelerate against gravity upside down from either manual input or override.

So far I'm not seeing the issue xD

The issue is that, according to the math, it should be able to fly with tons of lift to spare.

It sounds like the point Digi is trying to make is regardless of the reversers being there or not, the engines you have those reversers on, by themselves, are not strong enough to lift the ship. So either you may need to recheck your math or there may be some mass that you are not accounting for in your math.

Hmm. Maybe I had something heavy in my inventory? I'll have to check again.