I’m at render scale 100, never touched it in fact, and my selected resolution is 2560x1440.
What I mean is I fly at 2560x1440 and use DLSS Quality from yesterday, not that bad in fact and GPU breathing more quietly, even getting good headroom (GPU usage went from 85% down to around 55%, vsync ON, 60fps, smooth).
At 4K 2160p it’s too much for my RTX3070. I would like to have more resolution in between.
I would like a way to get DLSS with new settings like Quality+ or Quality++.
According my reading, developers can follow the rules Nvidia decided (what you call fixed modes), or not. It mean currently we have:
Quality 66.6%
Balanced 58%
Performance 50%
Ultra Performance 33.3%
Nothing prevent MSFS devs to change all %, or create a new mode like
Quality+ 80% or Ultra Super Performance at 25% (this one probably too ugly)
That’s because I can’t choose DLSR 1.78 from 1440p. x1.78 and x2.25 are always from the monitor max resolution, so 2160p (too much).
Not sure I’m clear enough 
edit: only way I didn’t try is to set the Sim a 4K 2160p and reduce DLSS settings at performance or Ultra performance, or reduce Render Scale (but without any DLSS). I’ll test that, but no real hope…