QEMU and KVM 
Useless details
Proxmox uses its own toolkit, called qm (built on top of those 2), while pretty much all Linux distros have libvirtd in their repos (which is just another toolkit, built on top of QEMU and KVM - you can switch VMs between those, albeit you need to create the VM resources manually and just replace the target disks if you’ve got raw or qcow2 vdisks, then you’re good to go, I did a mass migration from OpenNebula which was administering many CentOS boxes with libvirt).
Well, technically, you would be using 3 ports of the Protectli, 1 for WAN (OPNsense) connected to your ISP’s modem / router / switch, 1 for LAN with at least 3 vLANS (OPNsense) connected to another 1 port for Proxmox, configured with the same vLANS. Alternatively, you could use a managed switch in-between OPNsense port and Proxmox port, so that you can connect other devices behind the OPNsense router.
Edit: also, I think you can bridge the 2 Proxmox ports and connect an unmanaged switch to the last port for your normal LAN / other devices (or a router / AP / switch combo, preferably one with WiFi 6). I would not recommend this setup if you have smart devices in your network though.
I would still recommend using the device as a dedicated router, since Proxmox updates requiring reboots are more frequent than OPNsense updates, so that your whole network won’t go down just because of Proxmox, but that’s just me. I find it especially important when things break and need to search the internet and you can’t do that when your router is down.
Manjaro sucks ass. Arch / Artix are way better, but Arch is not without faults from time to time, albeit way less often than Manjaro (at least that was my experience with it). Artix may be more beginner-friendly to install, due to Manjaro openrc folks joining with the Artix team. To beginners, I’d recommend their default openrc variant, you can find more documentation for it (since it’s the default init and service manager for Gentoo and Alpine as well). Runit is also used by Void, it has ok documentation, but it doesn’t need much, since it’s very easy to understand for intermediate Linux users. Never used s6, it’s the new kid on the block, supposedly better than everything else according to its developer. I never used Artix, but I don’t see why it wouldn’t be just as good or better than Arch (because honestly, systemd does too many stuff).
Another personal rant about Manjaro
When I used Manjaro for about 2 years (?, I forgot), I had lots of freezes. I couldn’t debug them, because it was a hard lock most of the time. Sometimes it was because of KDE Kwin_wayland, all I needed to do was ssh from my phone and kill the display, then SDDM would come right up. Other times, I also had to kill SDDM before it worked. But most of the time, it was Manjaro just being garbage and freezing. Manjaro was the only OS that upon reboot / shutdown stage, would refuse to unmount my /home partition, which was just basic ext4, nothing special. Systemd would not time it out and force unmount, I literally waited ~30 minutes twice and ~40 min once and after those 3 times with Manjaro being an ass, I distro-hopped. I’m not a fan of distro-hopping, I try to stick with whatever I have for however long, but sometimes it’s just not possible.
It apparently is pretty standard in some enterprises, but I would still advice against it. Especially since the poor device won’t be able to hold a lot of VMs (maybe it can do with an OPNsense VM and lots of LXC containers in a pinch).
Not necessarily a bad idea. Just that you have to assume that the internet may go down more often than usual compared to if the box was dedicated to OPNsense - unless you run OPNsense behind another router and it’s just a test setup, then it’s fine. But otherwise, I’m pretty certain it wouldn’t be a wife-approved setup.
What CPU does the Protectli has again? I recall it being a 4 core low powered embedded APU or a Celeron quad-core (based on Atom cores)? If so, it won’t take you very far.
Yeah, that might be a rare instance. My gut feeling tells me that dhcpd might not be enabled and / or NetworkManager doing stupid stuff. But I could be wrong. Yeah, debugging Linux networking as a beginner is very annoying.
The BIOS does nothing other than verify that all hardware is present / works (POST) and load up an OS bootloader, so no. If you installed an open source firmware, then it should be even better for Linux.
If I needed a small and portable setup (which I probably will soon), as a beginner I’d sell all the chunky server, get a small NAS (4x HDD bays max, as compact as possible, preferably 2 bays and getting larger HDDs, since mobility is more important), something like the Protectli as a router, a 12 to 24 port managed switch, a wireless AP (or an old router in bridge / AP mode) and an i5 Intel NUC or more (at most 3) for Proxmox. You should easily fit all those in a backpack. An additional thing that won’t fit in a backpack, but keep my sanity in check would be a 700-1000W UPS. I would risk not having local backups and backing up stuff only in a remote location, but that’s just me.
As an advanced user, I’d replace the Proxmox NUC (server or cluster) with a Pi cluster and run LXD and k3s. Probably the NAS as well, in favor of a DIY NAS using the RockPro64 and a SATA add-in card. Going all ARM is not for everyone, which is why I recommend NUCs for portable setups for beginners.
I then put two 2470v2 10 core 20 thread cpus in it…and converted it noctua tower coolers and modified the intake and exhaust… its probably overkill, but the cpus barely get to 60c running prime 95… however, this gen of intel is vulnerable security wise from what I read, even if hyper threading is disabled. I kinda went overboard… I also am not so good with all the optional software you can use that ties in the iDRAC and system monitoring software and hardware… there’s a ton of cool stuff there that I haven’t fully utilized. I really hope to keep it, and maybe set it up as the primary server then used the ryzen 2700 with the asrockrack x470d4u with ipmi as the test server.
