check the manual for your motherboard for the led indicators.

Check which key combo enters bios - from the manual!

Strip down the machine to bare essentials, no disk, one stick of ram, remove the video card if you have onboard video, etc. Start trying to get the most basic things to work before adding the next thing

Try swapping the power supply Try swapping the RAM stick, in another slot, different stick, etc

From the manual:

The Clear CMOS button located on the rear I/O can be used to revert BIOS settings to default. To clear CMOS, turn off the power and remove the AC power to the power supply. Allow 30 seconds to ensure no standby power exists. Press and hold the Clear CMOS button for 3 seconds.

Did you do all of that? I have read elsewhere that reset buttons sometimes don't work the first time (or at all), so make extra sure you've cleared out all the power from the motherboard and CMOS. Some people have even had to completely disconnect the battery.

If you're sure that you're sure you've cleared the CMOS, then you should move on to testing the problematic parts one by one. If that CMOS step didn't fully discharge the power and force a reset to default, it could be that the saved settings got fucked up, and it's now trying to use corrupted settings (instead of the default fallback ones).

If that doesn't work, you might try to see if you can do a BIOS upgrade via flashback, which might reset your BIOS settings to default, after which you can maybe get in and do more troubleshooting.

https://support.nzxt.com/hc/en-us/articles/11652846587803-Updating-your-NZXT-Motherboard-BIOS

After shopping for solutions online, i cleared CMOS via the button on the mobo. I hoped it would either help the keyboard get recognised by GRUB, or at least deactivate fast-boot. But after powering the pc on again, my screen stays blank and the indication LEDs DRAM and BOOT are glowing.

I had to boot from a USB stick and regenerate UEFI entries after things like that. Though it specifically said it couldn't boot.

What does your motherboard's manual say about this pattern of LEDs?

Try booting a live OS and running memtest? (disconnect all bootable drives first)

Can you double-check your keyboard works with other devices?

Try fully disconnecting the power from the wall, and remove the bios battery for a while and then try booting again. Sometimes cmos reset buttons don't fully work. Assuming it does boot make sure you go into the bios and fix the ram settings. Ryzen 5000 series isn't as picky about ram as older ryzen systems, but you do want to make sure the ram speed and timings are correct because the default is rarely correct.

Yeah no idea