I’ve checked your instance, and there is no sign of malfunction: opening the realms from scratch in Realm Studio happens in a reasonable time (max 12 secs for the largest), have you tried it?
As you’re moving to MongoDB, do you have a Support contract? If you have, you should take advantage of it, as there are questions about your data & setup that may be inappropriate to have in a public forum…
That’s a significant test: if Realm Studio opens in a reasonable time, that means that the server is operating properly, and the issue is somewhere else. Knowing that shows us where to look for issues.
If the issue is not about the server (and this seems to be the case, from our point of view), then following you on a Support contract is the best option, especially if you already have migrated apps.
Legacy Realm has been EOL’d, and the current support is oriented exclusively to migrating to MongoDB Realm.
We could look further into this, asking further information, please note however that, as was already pointed to you, we’d need to ask for details you definitely don’t want to have public!