I recently purchased the ODYSSEY - X86J4125 SBC from Seeed and it seems to be working very well with OpenWrt. I have tested the network speed on the onboard Dual Gigabit Ethernet ports and they both reach Gigabit speeds as mentioned. I am also running the OpenWrt system offered by Seeed which already contains many packages. I am already running NAS, Plex Media Server, Adguard Home and VPN on it!
I came across another cheaper board which is this CM4 carrier board and maybe will give it a try later.
I also found out that it's pretty easy to setup the board out-of-the-box because the above OpenWrt system by Seeed already had the Ethernet ports configured. (LAN at 192.168.2.1 and WAN as DHCP client). So I only had to connect the Ethernet ports to get a working router.
So far I am happy with replacing my normal router with an OpenWrt router and exploring more possibilities with OpenWrt!
Isn't Archer C7 v2 grand? >0.5Gbit/sec WiFi throughput from 7 year old device. Sure, 2.4GHz is crap and routing performance is somewhat low w/o offloading but as an AP/smart switch, it is cheap and delivers.
I did a lot of searching for a cheap device that was widely available in the UK and settled on the old BT Home Hub 5A - it was standard issue by one of our biggest internet service providers so there are thousands out there. A few sellers on eBay are selling them used and preloaded with openwrt from £17 about 23 USD. It's easily capable of the 100Mbps down that my provider Virgin Media has as their basic package, but I understand the ADSL in there is a bit outdated - not a problem with a VM cable modem set to modem mode (internal router bypass).
I don't know how cheap something has to be to qualify for this topic, but for regular home/small office use I've done quite well lately with the Cudy AC2100 (aka wr2100).
The only caveats I've found are that it doesn't seem to like higher 5GHz channel settings, and the initial firmware install is a little annoying to figure out the first time. (You have to flash THEIR signed version of OpenWRT, then from there you can install a stock one.) I haven't done heavy stress testing of performance though.