SPCR

I guess the answer is "it depends on the window", but in general does it make much difference having a window or not?

I'm only thinking of a low end GPU which will run fanless most of the time, and a Ryzen CPU at stick speeds, so nothing really demanding. Just needs to be reasonably quiet, so my guess would be that having a window would make little difference.

It depends on the mfgr.

Is the plastic or glass thick enough/stiff enough/attached well so it doesn't cause it's own vibrations? If it is, then there will be very little difference between having a window or not. Fractal does a good job, as an example.

I was thinking about a Sharkoon M25. I suppose if worst comes to worst I can fix the window with some insulating foam or something.

The main difference is with a window, you can't have a dampening sheet on the inside. I don't think a window is make or break, but it is going to be quieter without one.

If you chose quiet components you shouldnt need a case to dampen the noise, a more open case that allows better airflow, should need less rpms on the fans, again really depends on what are you putting inside.

Sorry, but that's just ... simple thinking.
No, a more open case will not be quieter, as airflow and thermal dynamics are factors that affect cooling and they CAN be controlled, and so can be the other factor, noise.
A good case with a positive pressure airflow, yet with some sort of noise dampening or at least very heavy and well put together panels, will always be better than an open case where airflow is ... chaotic, let's say.