Will I be able to sync my Thunderbird accounts with K-9 Mail?
Yes. We plan to offer Firefox Sync as one option to allow you to securely sync accounts between Thunderbird and K-9 Mail. We expect this feature to be implemented in the summer of 2023.
So is this sync functionality already available? (Edit: Citation corrected)
Its been in testing for a bit. It got bumped from the last major desktop Thunderbird release and since Thunderbird releases used to be synced to firefox ESR releases its been a slow turnaround to get their schedule moving faster.