I've been very surprised what I can do hand held in fairly low light with simply a fast lens. Yeah, for super dim lighting you're still going to need high ISO or a really long shutter speed, but for even moderate lighting that you're likely to get in an urban environment I've taken many a photo with sub 4k ISO and a 1/125 shutter.
Worth considering the other A7 lines too, just because you mention night photography.
For example, I know the main differences between the A7r iii and the A7 iii are the obvious one, the A7r has double the pixel resolution, but less obviously, it also has a reduced maximum ISO compared to the A7. Given that's 30,000 ISO vs 51,200 ISO (and up to 204,800 expanded ISO) at the levels we're talking about might not matter for what you're trying to shoot, but it's worth considering.
I can't/don't know the technicalities but I know that a camera body you buy once and update it after 5 or more years so because of that I would go for the A7r III.
I'd say youtube videos can help you to understand the differences between those models.
Why are you only looking at full frame Sony bodies? Note that I'm not saying that's a bad choice necessary, but what is your use case? You're also going to want a quality lens or lenses and those will add more cost - especially for full frame.
If you're coming at this from a purely budget perspective, an APS-C DSLR body probably makes the most sense.
I want the larger sensor, not least for night photography. I love the tone of Sony images. I want mirrorless. My current setup is an APS-C Canon system (70D). I have never loved it and want to switch.
When you say tone it's worth mentioning that you're probably talking about straight out of the camera JPEGs. I've used Nikon, Sony, and Fuji bodies over the years and their RAWs all look very comparable. I share your opinion on Sony's straight out of the camera JPEGs, even if you and I might be outliers here. I find them to be fairly accurate/representative of the real world. My D40/D5300 were pretty accurate, but the Z6II was a bit warm (not that that's unpleasant). Fuji is a bit too stylized, but was a bit more real world IME.
Do you care about AF performance? If yes, the A7III and A7III are where I would start. If not, walk back to the A7II/A7RII. I would personally skip the A7R as it lacks IBIS.