Do you have space for a 1000mm server rack and can just buy some 13th or 12th gen Dell servers?
You literally can't go wrong with what every enterprise has used and subsequently decommissioned. You're likely not going to hit the bottleneck that an enterprise would. The servers are relatively cheap and reliable.
Just look on ebay, it's cheaper to buy without hard drives because you're just going to get better (and larger ones) anyway.
The 12th gen servers are dirt cheap and if you want something more "modern" the 13th gen will do fine they're just more expensive. You get a ton of cores and RAM available for whatever needs doing and management is pretty easy.
If you need storage R730xd has plenty of drive capacity and good raid performance. If you need GPUs for you can get the upgrade kit for the R730 and slap an old NVIDIA card(s) in there to hook up to plex. If you just need raw compute in a small package R630 is probably the ticket.
I'd recommend either getting a dedicated machine for your router or a server with more than 2 ports. Just because putting it in a hypervisor gets tricky when you need physical ports for lan segmentation. Depending on what you need you may need to find a 1u that has 4 ports. A switch that can do VLANs allows you to have a "router on a stick" but you really should break out your WAN from your LAN.
All of these run Proxmox perfectly well and are well supported. You can setup a cluster with a few machines and be able to easily create a Celph array or do HA if needed or you want to play around with it. They also have the ability to have 10Gig ethernet if you really need it/want it.
I have four dell 11th gen machines that all run Proxmox and it's solid, easily able to use all of the storage I have across the machines. I run Plex and multiple services off of VMs and containers.