I think there is solid reasoning in most of what you said. But there is a key point that I don't agree with...
As long as the dems are anywhere before the line, even if they’re shockingly close, then people will have to pick them. So they move right up to that line because they won’t be punished for it. In turn, Republicans have the space to move further right now that the overton window has shifted.
You say "people will have to pick them"; but that's obviously not the case - otherwise they would have won.
One could choose to reverse your reasoning to say that the Republicans are free to become more and more progressive without being punished, because there is no other real option. ... Clearly that doesn't seem to be happening; but why not?
There's are a lot of different reasons why the window might shift left or right... but one fairly simple force is that it will tend to shift towards the party that is winning. Right now, the Republicans are winning, and so the Democrat tries to be just a bit more like them in the hope of capturing enough for the 'middle ground' to win. It is pretty natural to think that the middle is somewhere between the two parties, rather than further to the left. I think if the Dems were actually winning elections then they would not be sliding towards the other party. It would be the other way around.
I reckon Lemmy is actually pretty good. I wouldn't know if the mobile apps are good, because I don't use those - but for me the core functionality of Lemmy on my computer is smooth and functional. I don't see any obvious low-hanging-fruit.