The only real alternative is using a Linux phone. The main problems with that would be (1) hardware support; (2) running Android apps. Although, (2) already has solutions in development or you could replace those native apps with web apps.
This is a comparison of popular Android “ROMs” (better term: AOSP distributions or Android-based OS). Please note I’m not affiliated with any of these projects and I am not giving any specific recommendation. If you think anything is factually incorrect, please let me know.
The only real alternative is using a Linux phone. The main problems with that would be (1) hardware support; (2) running Android apps. Although, (2) already has solutions in development or you could replace those native apps with web apps.
https://mobian-project.org/
https://sailfishos.org/
https://www.ubuntu-touch.io/