Migrating from kvm + drbd to incus

You can’t. Just at the database level, Incus uses Raft which is a consensus based protocol.
It’s not possible to get quorum/consensus with just two systems.

On the storage front, you’re going to hit a similar snag though we have some work scheduled which may help with that in the near future. Basically we’re going to be adding support for clustered LVM (lvmlockd) which allows for using a shared disk (whether through fiber-channel, NVMEoF or through DRBD) and then have multiple systems place instances on that.