Cross-tenant synchronization and resource access

You can control the user type as part of the sync, they don't necessarily need to be synced as guests.

You can have multiple instances of Entra Connect to separate tenants, the supported topologies are listed here: https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/plan-connect-topologies

Cloud sync doesn't yet support this, afaik, here's the corresponding documentation: https://learn.microsoft.com/en-us/entra/identity/hybrid/cloud-sync/plan-cloud-sync-topologies 

Whether the solution outlined above will do depends on several factors, such as the workloads and applications involved. My advise is to spin some test tenants and try to replicate the current and intended configuration as close as possible in order to discover issues early in the process.