Is there a way to use KMP like this? having expect/actual would be really useful for this project.

You can do it if

fabric

and

forge

are targets in a single gradle module, rather than their own separate modules. That might require a configuration like this https://kotlinlang.org/docs/reference/mpp-set-up-targets.html#distinguish-several-targets-for-one-platform

I need it to be separate projects because I need to use plugins one for forge and one for fabric and they conflict with each other.

yeah, in that case you're out of luck. If you want to control things from your common module, define an interface the old-fashioned way and implement it in the other modules.