While I like the form factor I’m unsure about the built in Ethernet and it’s suitability under openbsd.
I’d try out opnsense and if that fails to work I’d have a look at openwrt. Opnsense just feels a bit more polished but both are more than capable for what you talk about.
If you plan on setting it up as a router-on-a-stick remember you need vlan support on the switch it’s connected to.