Managing 4 separate Entra tenants after acquisitions and everything is a nightmare

Bought three companies in two years. They all had Entra tenants. Plan was merge everyone into ours but legal says we have to keep them separate until deal terms expire which is another year minimum. Four separate Entra instances that don't talk to each other.

Sales works across all four entities but each tenant has different licensing and different policies. Guy in Tenant A can't open Teams files from Tenant B without guest account that breaks inheritance and now he's got two identities. B2B collaboration sort of works except guest accounts don't get group memberships right and every shared folder needs manual permissions. Any policy change means doing it four separate times. Password requirements are different between tenants, MFA setup is different, it's all fragmented. People working across companies have multiple logins and keep using the wrong one. We're stuck like this until legal says we can consolidate and even then you can't merge Entra tenants you have to basically rebuild everything. Anyone found a way to make this less painful when you need cross-company collaboration but identity is stuck in separate silos?