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What are the "easy" steps to "overlay a separate read-write filesystem over the readonly system partition"? I would not even know where to start.

Just a simple line :-)

sudo mount -o remount,rw /

Then we can sudo apt install anything...

sudo apt install gcc gdb git make libgles2-mesa-dev

This is re-mounting R/W the root, not mounting an overlay. What you install this way will be wiped out on next update.

I think something like "mount --bind olddir newdir"...

This is a bind mount, not an overlay. For an actual overlay you need something like overlayfs or unionfs. See for example https://wiki.archlinux.org/index.php/Overlay_filesystem

Thanks! Will try that out