Honestly, pre-made containers are usually black boxes and also a huge waste of resources. If anything, your problem is not using NixOS or Guix, which means you have no reason to waste resources with Proxmox and maintain a massive attack surface thanks to ready-made containers from who knows who, maybe even with their forgotten SSH keys left inside, with dependencies that haven't been updated in ages because whoever made them works in Silicon Valley mode, etc.

You're making a lot of assumptions there.

First of all, I don't see how containers are inherently black boxes or a waste of resources. They're a tool to containerize applications, which can be misused as anything else. If you build your own images, they can certainly be lightweight and transparent. They're based on well known and stable Linux primitives.

Secondly, I'm not using containers at all, but VMs. I build my own images, mainly based on Debian. We can argue whether Linux distros are black boxes, but I would posit that NixOS and Guix are even more so due to their esoteric primitives.

Thirdly, I do use NixOS on several machines, and have been trying to setup a Guix system for years now. I have a love/hate relationship with NixOS because when things go wrong—and they do very frequently—the troubleshooting experience is a nightmare, due to the user hostile error messages and poor/misleading/outdated/nonexistent documentation.

By "black box" I was referring to the black magic that powers ZFS. This is partly due to my own lack of familiarity with it, but whenever I've tried to learn more or troubleshoot an issue like the performance degradation I'm experiencing now, I'm met with confusing viewpoints and documentation. So given this, I'm inclined to use simpler tools that I can reasonably understand which have given me less problems over the years.