Since Big Sur the OS root volume is sealed and boots from a snapshot for security reasons. To install the Kepler driver kexts into the OS you have to disable that security feature.
The thing is, the MacOS Installer will revert that each time a new update is installed. So every time there is a security update you have to break the seal and re-install those drivers again. And since Apple does not maintain them anymore, any future update of MacOS can render them useless and may even crash trying to load them.
It's a short term workaround, but if you want to use MacOS Monterey in the long run you should look out for a GPU replacement.