I use a Belkin RT3200 (same hardware as Linksys E8450). They go for pretty cheap on ebay and I have no complaints.

I do the same. I have Debian with KDE and Kodi autostart on boot. Use Kodi most of the time but can tab out if needed.

I’ve tried proxmox! Don’t currently have any hardware to run it but excited to do it. Part of the reason I don’t want to put it on the mini PC is that I can’t connect 3.5" drives over sata. I want to use the PC I plan on building as a combo server/NAS.

I am planning on a lot of services, but we’ll see about the rack of servers.

No, off the shelf routers are usually ARM and opnsense is x86 only.

There’s support coming for wifi 6 Qualcomm Atheros chips. The images are still in the snapshot/release candidate stage, so not a full release yet, but they are working.

There’s a lot of used mini PCs from Dell, HP, Lenovo that go for cheap on ebay. Those are a good alternative.