K-9 Mail joined Thunderbird to become Thunderbird Mobile [0]. They regularly publish progress updates [1] and their GitHub repository is regularly publishing releases and merging fixes and features [2].
What exactly do you want synced? Shouldn't the two mail clients just sync with your mail server individually? If you send an email from desktop, it will land in the sent folder, which the mobile client will then sync and show.
settings, calendar, tasks, contacts, I have multiple mail accounts and a lot of info that reside only on my main PC currently, I estimated that redoing everything would take hours to me.
0: https://blog.thunderbird.net/2022/06/revealed-thunderbird-on...
1: https://blog.thunderbird.net/category/thunderbird-mobile/
2: https://github.com/thundernest/k-9/pulse