I run a Unifi USG-1 router/gateway now but before that I got a used AC1000 router from the thrift store for $5 and loaded OpenWRT on that. I eventually got an Aris modem to replace the Comcrap Gateway because it was messing with the DNS traffic, even when piHole was set as DNS for every machine. So if I were you I would go look at the OpenWRT list of units that will accept their firmware. Pay attention to the hardware revision as well as the model number, it matters.
I’ve been looking at a bunch of different SBCs that can run OpenWRT because I really want that minimal power draw, but there’s so many more that are x86