I would like to somehow use a proprietary driver only for games, and for other conditional desktop tasks Nouveau.
I don’t think that’s possible, because of the way the drivers work.
If you have two separate GPUs, then you can indeed use one GPU for one thing and the other one for other things, but to the best of my knowledge, swapping GPU drivers for a single GPU at runtime is not possible.
Quick thought wihtout explanation …
maybe some modprobe or something could? I feel its possible.
No, I have tried that sort of thing myself already at some point in the past. It doesn’t work, because once a video driver has grabbed hold of a particular GPU, it won’t let go of it again, even if you attempt to force-unload the driver module.
I believe it’s due to the fact that the Linux console as built into the kernel is not multiplexed, and once it grabs hold of the primary GPU ─ i.e. the one used for booting the system ─ it remains bound to its PCI(e) address. 
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Then what is the best way to switch between drivers on reboot?
Wouldn’t it make more sense to just use the intel iGPU for normal tasks and for games the nvidia (with proprietary drivers)? like video-hybrid-intel-nvidia-455xx-prime
My i5-3570k does have some video core, but it’s not even running or connected, I don’t know, didn’t think about it. Can this be done normally, where to start?
I have no experience with nvidia, but I guess so.
https://wiki.manjaro.org/index.php/Configure_Graphics_Cards#nvidia-prime
At least it sounds easy:
Simply select and install an 'nvidia-prime' profile through MSM or mhwd (such as *video-hybrid-intel-nvidia-440xx-prime* ) and ensure the package *nvidia-prime* is installed.
I guess you’d want: video-hybrid-intel-nvidia-prime
mhwd does not offer to install nvidia-prime. I think Intel’s video core is down or disabled, I don’t know.
Yeah, it probably only works on laptops with Optimus.
I’ve never tried to make it work this way, but the steps must be more or less as follows:
- Connect your monitor to both iGPU on the motherboard and dGPU with 2 cables.
- Enable iGPU in BIOS.
- Boot and see what’s in charge now: Intel or Nvidia.
- Backup config files from /etc/modprobe.d, /lib/modprobe.d and /etc/X11 to files with a .bak extension.
- Install an opposite config through mhwd.
Check what configs have been modified. - Reboot and pray. If it worked, phew. When you need to change a driver, use mhwd to change to the opposite.
- If it didn’t work, well, just restore the original files, or, again, use mhwd.
It’s not a how-to of course.
I can’t imagine how can plug two cables into one monitor connector. Or what is it supposed to be?
I think the idea is to connect both, the iGPU out and nVidia out into your monitor (which i guess has more than one input port, be it dp or hdmi; mine has 3 for example). And then the monitor would switch based on which card outputs a signal. However, that’s sound like a quite adventurous workaround. 
My thought was that it is possible to pass through the signal of the dGPU to the iGPU port, similar to what the Optimus laptops are doing. But that does not seem to be possible I guess.
Yeah, I meant both. My one, for instance, has 2 HDMI ports and 1 VGA.
Well, strange ideas lead to strange workarounds.
Penguin is a curious creature, after all 
Bad luck. Stick to 1 driver then.
Or, you can try the above method (slightly modified) anyway.
I mean each time before a reboot install video-modesetting with mhwd when you need nouveau, or install video-nvidia when you need nvidia. Reboot is a must, but you know what? Why not try to swap to AMD card with somebody? Penza is quite a big city you may find a person who needs Nvidia card for some reason. Since you’re not using a laptop and your GPU is not soldered, this is an easy solution. RX 550 or something like that could be a good choice I guess. OK, I’m not an expert in GPU specs and so on, so let me leave you some room for imagination. Trivago, oops, Avito, local forums, spec check sites, etc.
But why switching at all. Just use the prop nvidia ones.
Or does nouveau use less power or something?
I wanted to eliminate this problem:
In any case, apparently, have to get something from AMD.