How do you handle IAM when your company acquires another one every 6 months?

We've grown a ton through acquisition, and our IAM strategy is buckling under the pressure. Every six months, it's a new company with its own Azure tenant, a different IdP, and a mess of custom apps.

I'm trying to move us toward a single source of truth, but the constant mergers make a "big bang" consolidation impossible. Right now, we're stitching things together with SCIM and some Python automation for account provisioning, but it feels fragile.

How are you all handling this? Do you enforce a "you must join our tenant" policy immediately, or do you run a multi-tenant federation setup for a transition period? I'm especially curious about practical experiences merging directory attributes and handling access reviews across disparate systems.