By following the 340 dkms drivers installation provided on philm’s GitHub, you couldn’t have it to work? That’s weird.
For me the 390 drivers currently provided are half broken, but I got no response on the thread I opened… but the 340 drivers work perfectly on friends computers for old Nvidia cards.
Before leaving, maybe try that, clean system from any previous Nvidia, follow the guide, then make sure to blacklist Nvidia drivers from updating if you have the AUR enabled as it would try to update from the AUR (what you don’t want with this DKMS installation guide provided by philm).