One more project worth checking out is tpikonen/ols: Offline Location Service - Codeberg.org, though I personally haven’t tried it yet. You could set up an offline database of your provider’s cellular towers to let you get at least an approximate location fully offline (as long as the modem is on).
IMO, Purism should at least set up an Ichnaea-compatible endpoint for retrieving OpenCelliD-based location and use that by default in PureOS. That won’t get us WiFi-level accuracy, but it would be much better than status quo.