Update: I wound up getting one of the Chinese mini PCs from Amazon. $300+tax got me a Ryzen 7 5700U, 32GB of RAM, 1TB NVMe storage, and a single 2.5Gb ethernet port. I can add a second interface via USB-C if necessary. Really not bad at all. I have Proxmox up and running on it already, with PiHole and Jellyfin already running in LXC containers. Jellyfin took a bit of screwing around to get the CIFS shares from my NAS and hardware-accelerated transcoding going, but everything works now!
homelab
I got a few HP Elitedesk/Prodesk computers. One with an i5 9500T for ~200€, a couple with i5 6500t for about 100€ each (one small factor, the other a bit larger, with a PCI slot for a GPU). Not the most recent or powerful, but more than Enough for a homelab with a handful of VMs. Power consumption sits around 36-50w for the 3 machines, a small dlink switch and a Synology NAS with 4 drives.
I am a fan of the AooStar R7 at the moment.
As you have the Nas you don't need the 2 bays populated but it comes with 2 nvme slots and capacity up to 32tb of ram.
Find an old Thinkcenter on ebay.
Do you have any specific price/model recommendations?
Yeah, this is my recommendation. I have a cluster of 6 little Lenovo Thinkcenters of varying ages. Some of them have been about the price of an rPi too.