Here's some potentially good news about an alternative...

There's an interesting comment on this Mozilla Github post about Graphene OS' plans in this area, it's quite a long comment so I'll link it. The first para reads:

GrapheneOS Foundation has been planning to host a network location service for GrapheneOS and projects collaborating with us for a while now. We've received significant funding we can put to use for this to make a high quality, modern implementation on both the client and server side. A new unified app (cellular, Wi-Fi, Bluetooth beacons) for gathering data to publish as fully open data could also be part of it. We also plan to make a SUPL implementation as part of the same service as an alternative to our Google SUPL proxy to replace it as the default in the long term.