I don’t think any alternatives exist that are easily viable.

Most require you to buy specific phones and flash the OS or buy them from a specific supplier who pre-images them.

Carrier compatibility is also a major issue if you’re in any country with a cellular network requiring that you use VoLTE services in general…these third party OSes can’t always get at the vendor binary blobs or simply won’t include them because legal and privacy preserving reasons.

You may have some luck with GrapheneOS or maybe LineageOS but those are the only mainstream “alternates” to Android that exist; which actually have a decent chance of working well and can usually be installed to a fair number of different devices; as long as you can unlock their bootloaders and root them day one.