You can be on the development team if you like. 
Duplicati needs more volunteers than it now has.
That’s basically the reason progress is slower than some may like. It’s not just programming though.
How the backup process works might be of interest. The reason I’d think it will do Thunderbird well is Compacting folders where it sounds like new messages (at least for POP3, not sure about IMAP) get appended to the mailbox, and deletions are flagged but are just a local edit, not a massive file rewrite. Periodically the compact does that massive rewrite, and at that point the backup of changes gets big.
I believe with a Cobian incremental, it’s always whole files. With Duplicati, it’s incremental at 100 KB (default – large backups may like larger) deduplication blocks, so fits Thunderbird between compact.
It’s (I believe) a more complex backup scheme than Cobian has. It has its advantages, but exception handling has a lot more potential failures to worry about, and field-debugging is basically not practical.
https://usage-reporter.duplicati.com/ shows over 3 million backups per month, and forum sees some problems come up. The repair tools (if needed) usually get the job done. It’s very rare to lose backups completely, however sometimes one winds up starting fresh, and old versions of the data may be lost.
Best backup practice is multiple backups, including something remote. Cobian might still serve some.
EDIT:
From - Sun Apr 25 09:58:24 2021
X-Mozilla-Status: 0009
demonstrates what I mean. That value is a hexadecimal display of flags, and by deleting a message in a local mailbox I changed it from 0001 to 0009, basically setting the MSG_FLAG_EXPUNGED flag in-place.