Cross-Tenant Shared Meeting Room Spaces

It's really up to you. Enable the room mailboxes to process external meetings, and go from there. If you want the room's visibility to be exposed cross-tenants, you need to configure an org relationship (sharing policies are mostly targeted at end user scenarios): https://learn.microsoft.com/en-us/exchange/sharing/organization-relationships/create-an-organization-relationship

For the purpose of processing external meetings and fetching free/busy it is not necessary to have the room objects synced across tenants, but if you want to expose them in the GAL, you can provision contacts. Keep in mind that the end user experience for those will be different from adding room mailboxes in the "home" tenant, and you cannot add them to room lists either.