Apt configuration - As Stable As Possible

Hi all,

I am trying to configure Apt to follow a "stable where we can, unstable where we must" logic.

On the "Stable+Backports - Testing - Unstable - Experimental" stencil, I would like to

- install left-to-right (the stablest version available),

- upgrade right-to-left (the stablest newer version available, but never less stable than the one currently installed).

I have already achieved the desired Install behaviour with Pin-Priorities, but upgrade is more challenging.

For example, if I have foo=2 installed from testing, and the newer versions available are foo=3 in stable and foo=4 in testing, apt seems to go for testing, whereas I would prefer the stabler though not-so-much-newer version by default.

Furthermore, I would like to keep the aforementioned stencil fixed on my current release until and unless I'm ready for an upgrade.

I imagine this can be achieved by sticking explicit names (e.g. bookworm/trixie in place of stable/testing) everywhere, but maybe there is a nicer, DRY-compliant method? Perhaps, something involving APT::Default-Release or similar, though I will admit I have no clue how this setting actually works...

Hoping for advice/clarifications, thanks!