Apps in App Manager are extras packages from the Leste community.
MyPaint is a Devuan repo app, so absolutely not optimised for your device.
Zram is not needed, and not activated (even if you see messages about it), no worries
I recommend you to just dist-upgrade to -devel:
Add in /etc/apt/sources.list:

Then 'apt-get update' then 'apt-get dist-upgrade' and reboot.

If the device become responsive with no lags, update is ok
If not, something is probably wrong with the device or the SDcard.

Note: fast battery draining and unexpected reboots are not 'normal' and not known issues AFAIK. Maybe your battery is faulty.