Do you sync logs with DJI? If so then the problem is syncing will automatically replace missing logs.
If you do not sync then connecting the DJI RC to a computer via a USB cable should allow the computer to delete logs, however I would suggest it is better to move logs to the computer etc. rather than delete them.
Also note that as well as the txt flight logs there are or should be .DAT flight logs in the folder whose name starts "MCDATFli". It is likely that they take up more space that the .txt flight logs, with the
mini 3 it is possible that those .DATs are readable/processable, if correct then I would be inclined to move them to your computer as well.
You might also be able to delete logs via any file browser that exists on the DJI RC controller.
BTW syncing seems to remove the .DAT so if you have sync'ed you may not have DATs.