The old laptop is the same one I use for all computing. So using an SBC would just add to the energy consumption.
But an SBC could be interesting anyway because there could be moments when I would want a phone to connect without the laptop dependency. So I would be interested in hearing how it works. Does the SBC also charge the phone over USB? Does the reverse tethering software exist that can run on an SBC? It would be cool to have this configuration:
phone → USB → SBC → ethernet → router…
Especially cool if the SBC could run Tor and proxy all traffic over Tor (though I suppose that job would best be served by the router).
Considering the price of a Fairphone you have a right to be upset. I suppose the only thing going for your situation is maybe the parts are worth something since they can be used to easily fix other Fairphones.