Inkpad X has become unstable

[jkaltes]

28 April 2020 I have bought an Inkpad X. It was always very slow, but after some updates it has become much slower. Earlier it booted reasonably fast, but this has totally changed.
Beside that it is now much more unstable than before. Earlier I had to reboot maybe once in 5 days, now I have to reboot constantly. At such occasions holding the left button doesn't show the task list; nothing seems to work anymore, the only remedy is a reboot.
I never use the "front" light and earlier I could read for days the whole day, without needing to recharge. Now when I start reading with fully charged battery I have to stop reading after a short period time and connect the reader with a charger.
After a factory reset it was as slow as before, but it seemed to be more stable, but now it is even less stable as before the factory reset.

Is there a more fundamental reset than the 'Factory reset' I have used before.