This, seriously. The long-term maintenance, tribal knowledge & risks associated with this giant hack will be greater than anything they'd ever have expected. Inb4 global outage post-mortem & key-man dependency salaries.

There's no virtually no excuse not spinning up a pg pod (or two) for each tenant - heck even a namespace with the whole stack.

Embed your 4-phases migrations directly in your releases / deployments, slap a py script to manage progressive rollouts, and you're done.

Discovery is automated, blast / loss radius is reduced to the smallest denominator, you can now monitor / pin / adjust the stack for each customer individually as necessary, sort the release ordering / schedule based on client criticality / sensitivity, you can now easily geolocate the deployment to the tenant's location, charge by resource usage, and much more.

And you can still query & roll-up all of your databases at once for analytics with Trino/DBT with nothing more but a yaml inventory.

No magic, no proprietary garbage.