Confirm approach and architecture to stay decentral(cross tenant sync) or to merge tenants

Hello,

We are a group of companies with a decentral setup (all small companies 15-200 users). We are planning to implement a central ERP (D365) system for all the companies. I would like know/validate my options and know if the proposed solutions are possible.

1. We stay decentralized. Meaning each entity has his own AD -> Entra ID-> M365

To allow them to connect to a central applications like D365 we could invite the entities to the HQ tenant as external members by using Cross-tenant syncronization.
https://learn.microsoft.com/en-us/entra/identity/multi-tenant-organizations/cross-tenant-synchronization-overview
Or to wait for MTO to be released

https://learn.microsoft.com/en-us/microsoft-365/enterprise/plan-multi-tenant-org-overview?view=o365-worldwide

    2. We take the first steps to Merge all the tenants. First step would be to Create a separate user account in the HQ tenant for each user and keep them in sync. This way the users will have 2 accounts. There original account and their new account to access the central ERP with the idea to migrate services in a later phase to this account. To do this I was thinking to implement the following:

It is a combination of 2 supported scenarios:
https://learn.microsoft.com/en-us/entra/identity/hybrid/connect/plan-connect-topologies
Would this be possible?

Or could we use Cloud sync to accomplish this?

Thanks!