Hi just did a test if this boards are upgradable. Now I have 2 dead units.
ROS5.21 and 6.0rc1 killed these. So I have to stick with 5.6 in the field
until they get replaced.

Why every newer OS has to eat more resources doing the same?
Shouldnt a newer better OS eat less resources?

you need to upgrade routerboot before you upgrade routeros.
you find it here:
system -> routerboard -> upgrade

if you already bricked your rb112/133 you can revive it by downgrading it to 5.6 with netinstall.
afterwards upgrade the bootloader, then upgrade routeros.