AREDN can give you a mesh running standard internet protocols on fairly cheap hardware - they have firmware that can be flashed on small USB-powered GL-iNet devices such as Beryl:
What would be lovely, is a giant repo of "every tool, firmware file, and documentation you might need, to reflash any random consumer router you might come across", completely offline and packaged up.
Because inevitably, you find a bunch of them after you've lost access.
https://www.arednmesh.org/content/supported-devices-0
It's self-configuring, too - as soon as the node spins up, it will automatically find and connect to nearby nodes and start routing.