How to handle realms?

I run 2 similar services on different domains.
And maybe both services will be transferred to different organisations (non-profit organisations) somewhere in the future.

And now i want to know, how to handle such a situation?

For now it would be a good idea, to have both applications in the same realm on keycloak. So users have a SSO for both.

I want to know, if it is possible, (and easy!!!) to transfer a bunch of users to another realm or even another kc-instance?

Is it good practice, to separate services in own realms, which run on different domains?

If migrating users without pain to other realms or instances is possible, then i would put them all together in one realm for now.

And can i mix users in the same realm the just “live” in keycloak only an other who are mirrored from a ldap?