So if I stick to negative film, and around 1m, I should be good? I'm considering buying a Metz 45 series eventually.
You'll still get a lot of underexposed shots as your subjects are more than 1m away. You have to try around a little bit. You can do that with the DSLR, just put the flash and the camera in manual mode.
A wide for the Nikon sounds good, if I could afford it. How do superwide SLR lenses differ from RF lenses in terms of light fall off, sharpness, and distortion?
Less than they did in the 1960s. In general I'd expect a good RF wide still to be better than a SLR lens, but SLR wides are pretty decent nowadays. Regarding the Tamron 17/f3.5: it's a decent lens, not the best in the world, but I got it (in the Canon FD version) for under 100 EUR, plus an Adaptall mount for the Nikon. There is simply no RF ultrawide for that kind of money, much less one that is better:
It holds up quite well, even when shot directly into the sun, corner fall-off is OK for a 17mm lens, and distortion is pretty well controlled, too. It's no Hologon, but then it costs 100 times less (and is two stops faster
😉).
In fast club shots with a flash I wouldn't worry too much about light falloff, sharpness and distortion anyway. If you want to use that flash, getting correct exposure is a lot more important, and framing is easier, too.