Do you need to upgrade your router? Most of the time the bottleneck is the ISP limits on the Internet connection, not the modem or router. If you do want to replace your router, my vote is Opnsense. You can run it on pretty much any x86 machine with at least two Ethernet ports.
If you're not doing much computing, then the idle power consumption is much more important than the tdp, in which case the motherboard and other hardware are more important than the CPU. For your use case, something like a tinyminimicro 1L PC or a Celeron mini PC would be good. I've personally played around with
OPNsense will do what you want, although it's probably overkill in this case. Also keep in mind that any switching it does will be purely software, so it might struggle to push heavy traffic through, especially if you add rules and vlans and such. Openwrt is more lightweight and should also work, or you could just install a general Linux OS like Ubuntu or debian and use iptables to route stuff. The better option though is to just get a proper switch, since those have hardware switching.
Most of these boxes are just Celeron or Intel i3 based computers with a few extra NICs, they'll run pretty much anything just fine. They don't have actual switching hardware, but they've got enough CPU juice to push gigabit connections with a few simple rules/filters. I have an n5105 based one with six i226 NICs, with a simple bridge it can throughput close to 2.5gbps in iperf.
I like the Sonoff S31 smart outlets flashed with tasmota, they're super easy to flash, super easy to configure, can communicate through mqtt (also easy to configure), have switching capability as well as measuring, and cost $7 a piece on Amazon.
Mind sharing some details on that ryzen server? It sounds cool
I'll check that out, thanks!