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Is it? I guess you need mutable + persistant mount for /var and one for /home. /tmp is already tmpfs by default. All you then have to do is make the other mount points ro in your fstab.
And how do you then run apt upgrade?
(The answer is to write a script that mounts / rw, runs the upgrade, then mounts it ro again. But figuring out the edge cases isn’t something I want to get into.)
This is part of the maintenance. The workflow here would differ depending on numerous factors. An automated update sounds like a bad idea.
All I was saying is that setting debian up for immutability is more straightforward. How you maintain the os from there should already be known to someone opting into it.