For me it came down to an epiphany: I'm doing all this careful setup on a tricky, esoteric platform to get reproducible builds, except actually, some of my containers have to run Debian because I can't build the software they run deterministically. So now I only sort-of have reproducible, immutable builds.
So I'm jumping through all these hoops, stepping so far out of the norm that I become the effective maintainer and debugger of all of this, and ... Why was I doing this again?
Oh yeah, deterministic builds and immutability. And I needed these ... why?
Turns out I can get a similar effect on a mainstream platform with some scripts and a little bit of discipline.
Proxmox has its own issues (they all do), but it's a much more inviting experience and nicer community.