The project here in Montreal did their own openWrt firmware, but it’s deprecated. I think it’s the way to go.
Yeah there are things like the Google WiFi and Eero devices that use meshing for extending WiFi within a home but I think both are limited to 3 devices in the mesh so not suitable for this application. From a quick search I found LibreMesh which appears to be a mesh firmware based on OpenWrt so should run on a variety of cheap consumer grade routers. That might be something to look into. https://libremesh.org
Discussion
Maybe we should start a community founded project for Nostr Mesh Nodes on the same principles as today's Rpi based Bitcoin nodes work. Maybe it will help to decentralize Nostr relay nodes even more.