My first suggestion to you is to experiment with the trial version. TB! is unbelievably complex, compared to other email clients because of its many features. I've been using it for three years and routinely discover some feature I had never heard of. My second suggestion would be to convert to IMAP to prevent future data conversions. You asked many questions, so I'll just share a few thoughts:


The product is well-supported on functionality, but that does not mean it is supported well. If you read back through the posts on the forum, you will see that the upgrade to version 9 omitted many features that were in version 8. Tech support says they will be reinstated, but why removed? You will also see that when Google did an unanticipated change to Gmail. version 9 quickly had the fix, showing that basic functionality is a high priority. But there is frustration that previously enjoyed features are still absent. I accept that as a growing glitch; TB! gives impression of new features in future and I'm patient.


The company appears to be small, somewhat of a 'family' environment between staffers and users. All good. Tech support is prompt and always responds. Being a small organization, there is always risk of its ongoing stability, but it seems strong in its customer base. Like you, I believe in being fully involved, installing all paid upgrades to help ensure ongoing development and support.


The mail structure is fairly straightforward: all user data is in one folder with subfolders, separate from the software. I keep mine in a folder off the root drive and TB! can do backups on a predefined schedule. I also do daily backups of the folder to an online service. To aid that, I created a simple utility, available at https://davidkirk.org/thebat/ that I find helpful.  

Conversion is never easy. I used Agent 20 years ago, but don't remember format. My suggestion is to investigate whether Thunderbird and the import/export addon might assist, because if you can migrate to Thunderbird, you can from there easily move folders from POP to IMAP, a feat that I don't think other clients can do. That would ease the transition.


BRIEF UPDATE: I noticed this morning that the major developer is leaving the company immediately. I have my opinion on why that happened, but it relates to one of your questions on company stability. Something to consider. This might be a window on a great new future, or it may not. We will know in time.


Good luck.

david

Maxim posted his farewell message in the TBBETA mailing list for beta testers.