I can't say much to docker in LXC as I'm not using it, I vaguely remember some limitation I've read of but if it works fine for you those don't seem to apply.
A VM has more overhead than an LXC, but with several LXCs maybe a single VM wins on overhead.
I currently have most Docker containers in one VM and am thinking about splitting it, the main reason is that 2 deployments have way larger volumes than the rest. This leads to the snapshots of the VM being very large as well and if I would need to restore from snapshots for a "small" application, it would take super long because of the large ones.
A single VM may be a bit easier on maintenance than several LXCs.
If you don't have a specific reason to switch, I would not.
I was thinking about putting it from its dedicated VM to opnsense as well. I just don't know yet what the security implications are and also my firewall hardware isn't too beefy so I have to play around with it for a bit.
Intel CPU RAM limits often are wrong for some reason. If a Mainboard coming with that CPU supports more, it'll probably work. I usually try to search forums to see if someone uses the same configuration and how much RAM they got to work.
I'm personally using Prometheus Stack and like it, but I just check Grafana in my Android browser. I think Zabbix has an Android app but I don't know if it has as many possibilities as Prometheus.
Debian is kind of the default for many things now, so many guides etc will assume a Debian-like os but if you are familiar with other OSes you can try them just fine. Make sure zfs is supported by the OS properly if you want to use it for your SSDs (which I suggest).
If you are using Docker containers, you can migrate between OSes later relatively easy. Depending on how beefy your optiplex is, you could also use Proxmox as the base os and play with different VMs. Being able to easily snapshot the VMs is pretty great and for me has always been worth the overhead.
If you just want to install some apps directly on the server, I'd just go with Debian headless and set up docker, if you like with portainer or some similar GUI.
I don't know them so I can't say much. Personally I'm wary if there's too much "magic" involved, tools that have automated everything usually are easy to get going, but often tracking down issues is more complicated as they add another layer where the error can come from. But as I said I don't know these so I can't say much about them.
Oh wow, I hadn't known pinepods! I've been looking for a selfhosted podcast management thing for literal years and recently audiobookshelf popped up, but maybe I should check Pinepods instead! I don't have audiobooks anyways. Are there Android Apps that can be used as a frontend?
Btw, github links to https://pinepods.online/, but the website seems to be exclusively available on https://www.pinepods.online/.
Edit: Just found you release an Android app as well. I'll have a look!
I can't say much to docker in LXC as I'm not using it, I vaguely remember some limitation I've read of but if it works fine for you those don't seem to apply.
A VM has more overhead than an LXC, but with several LXCs maybe a single VM wins on overhead.
I currently have most Docker containers in one VM and am thinking about splitting it, the main reason is that 2 deployments have way larger volumes than the rest. This leads to the snapshots of the VM being very large as well and if I would need to restore from snapshots for a "small" application, it would take super long because of the large ones.
A single VM may be a bit easier on maintenance than several LXCs.
If you don't have a specific reason to switch, I would not.