I think openwrt on the main unit (a good router) + nodemcu based sensor units would work well.
Openwrt is powerful enough to take care of logging, connecting to the internet etc, and the sensor units can form a pseudo mesh using their wifi capabilities.
Ideally the base should be from a project that handles base system upgrades, so that consumers don't have to trust that the vendor is building security updates in a timely manner.
Last I checked, OpenWRT did not do security updates, and it's up to the user to recompile everything if they want newer versions.
Openwrt is powerful enough to take care of logging, connecting to the internet etc, and the sensor units can form a pseudo mesh using their wifi capabilities.