The fixed foreground versus moving stars is a problem I know well. You basically need to process it twice - once for the stars, once for the foreground, then combine.
The question about simulating a long exposure is really a daytime-only issue. If you want to get a certain dreamy look on moving water, you can use a really long exposure. Another scenario is you want to shoot a street scene but have the people disappear.
One way to do this is a 10-stop or 15 stop neutral density filter. But those are a pain to use (you can’t see through them!), so must be applied after composition and you must use care (and black gaffer’s tape) to make sure there are no light leaks. You need to have an intervalometer because most cameras won’t time a 10 min exposure for you. And you need to have the filters and intervalometer with you at the time.
An alternative to a single long exposure is to take multiple shorter exposures and then average them.
My question was about what combination of exposure gives a similar time integration effect. That isn’t about SNR it is about how fast things move in a frame and how it winds up looking.
In those cases SNR is not really the issue - you have plenty of light.
There is no good software for doing image averaging of terrestrial shots so Siril would be a very valuable tool for that. I was excited to see that Entropy512 has tried this and was asking him about the various trade offs.