beaconDB is a drop-in replacement for MLS, which uses the same format request that’s used by Mozilla’s Ichnaea.
The source code is available on Codeberg: https://codeberg.org/beacondb/beacondb
You can contribute to the project by using an app like NeoStumbler (GitHub) or Tower Collector (GitHub) to submit location reports. NeoStumbler does Wi-Fi, Bluetooth and GSM, while Tower Collector can only do GSM cell towers. Both are FOSS and available on F-Droid.
It is also recommended by the GrapheneOS project: https://grapheneos.social/@GrapheneOS/112759509558471713
https://grapheneos.org/articles/positon-location-service
Just keep in mind that it’s still in relatively early development, which is why it really needs contributions.
Using a location service obviously means that this service is going to know your location. beaconDB already minimizes the data that is collected about users. There’s not much else that can be done to make these kinds of services more private. The other options (Google and Apple) are much worse. The only alternative is not using a network location service at all, and simply relying on GNSS + PSDS and SUPL, like GrapheneOS does by default. I’d say beaconDB is the next best option, much better than proprietary alternatives and on par with the now defunct Mozilla Location Service.