Hi Aleks, thanks for your reply. I managed to get the migration to work before I saw your response; I think the failure may have something with an incomplete removal of firefly-iii causing issues. In addition to a couple reboot cycles, I did the following:
sudo dpkg --configure -a- Delete
/etc/php/7.4/fpm/pool.d/firefly-iii.conf - Install
php7.4-fpm - run
sudo apt --fix-broken install - run
sudo yunohost tools migrations run --accept-disclaimer
after which the migration was successfully completed.
For context: I have had strange issues ever since installing firefly-iii (such as being unable to install or update apps), which was many updates ago. When experiencing these issues some time ago, I removed php7.4-fpm after finding this thread, which appeared to resolve them for a while.