Hi,
Did you explore the Cross Tenant Sharing feature? I think this should work in your favour as the Cross tenant feature is for this kind of requirements and scenarios - https://learn.microsoft.com/en-us/azure/active-directory/external-identities/cross-tenant-access-overview
Also check this - https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/multi-tenant-user-management-introduction
Please don't forget to upvote and Accept as answer if the reply is helpful