Have you tried the DRI_PRIME=1 variable?
What does DRI_PRIME=0 glxinfo|grep OpenGL and DRI_PRIME=1 glxinfo|grep OpenGL say?

Quoting: tuxintuxedoWhat does DRI_PRIME=0 glxinfo|grep OpenGL and DRI_PRIME=1 glxinfo|grep OpenGL say?

Both come up as NVidia...

But from a quick look on google (and the Arch Wiki), would appear that DRI_PRIME is what will be needed... guess I'm probably missing some small piece of the puzzle.

I think that nvidia uses different environment variables: https://download.nvidia.com/XFree86/Linux-x86_64/435.17/README/primerenderoffload.html

You will have to figure out the identifier of your intel gpu in your system though.

Keep in mind that you need to make the iGPU the primary display device first. The intel driver is not hooked up to support DRI_PRIME in any other way.

This does seem like a very roundabout way to get the game working though. What's the actual problem? The issues in Windows shouldn't impact Wine given that one uses straight D3D and the other OpenGL.

Ahhh, iGPU needs to be primary... that'll be why I've not had any success so far! (I did wonder if that was the case, from reading some of the DRI_Prime info)
Will fiddle around with it once some (elusive) free time appears.

As for what the problem is - performance. Game itself will run with either GPU, but I get 20-30% better performance (measured in terms of the game's "Processor Occupancy" (PO). On the intel chip, at the start of a race, I get 90-100% PO (which is great - runs at full speed), wheras on the Nvidia card, I get 120-130% PO (which leads to annoying slowdown and some physics oddities. Wonky late 90s game engine ftw)

Now I could run it in software mode (and get around 15% PO), but many track mods require hardware mode (and also, software mode seems to be a bit more crashy in Wine for whatever reason). But this is Linux - why settle for a lesser experience when tweaking is possible!

To be fair, the fact it runs at all is fantastic. Hasn't worked well in Windows since XP came along (and even then, only worked well if you had an older ATi Card - NVidia cards give texture bugs)

Another alternative, if you like fiddling with stuff, is to use the Zink driver to get a Mesa OpenGL driver on top of Nvidia.

A third options is the llvmpipe OpenGL Mesa driver, but only if you have really beefy CPU. Probably not a realistic option with CPU utilization being a problem with the game.