I took your question to our weekly Thunderbird on Android/K-9 Mail design meeting today. The answer to the question “when will the old account overview screen return” is: possibly never.
We do want to add some of the things the account overview screen enabled, e.g. faster switching between accounts. But those features will most likely come in a different form.
For now we are working on other features, though. You can have a look at the roadmap here: Android Roadmap - Thunderbird
If you really can’t live without an account overview screen, you should probably switch to a different email app. FairEmail is also an open source email app with a focus on privacy. It can be configured to start with a list of accounts.