PgAdmin4 unusable when database has lots of schemas
Yes, it will be because some of the system views will be getting extremely slow. No, switching tools is unlikely to help. There have been updates to how the system views are generated over the years to improve performance with complex databases. But 60,000 schemas? Not even tables, but schemas? That is so far beyond what 99.9% of users run that you are unlikely to find much help.

If you are committed to this approach, you should probably subscribe to the pg mailing lists where you can have an extended discussion about the beat way to manage your database.