Physical Access Control - Advice needed - multiple sites/multiple systems

So I work for a company that has a bunch of offices all over the map. Almost every office has a different access control system, and most of them are controlled by the office building management, not us. Some hand out badges or FOBs, some use an app. This concerns me, because we don't have a good system to cull users when they are terminated, etc., and I started trying to figure out how to approach this. I have an idea for a pretty ambitious software project if I wanted to go the DIY route. But I thought I'd try and see what others have done in this situation.

Has anyone else had to deal with multiple access systems they don't control? How do you unify these systems so they're all tied in to LDAP, etc. to verify a user's status? Part of the problem is that even if we came up with our own solution, people will still need the building's badge to get in the main door, parking garage, etc. And conversely, the building needs access to our offices for cleaning staff, etc..