The S905X3 enjoys decent community support, primarily from Russian and Chinese developers. The most prominent firmware families include:
Arguably the most popular custom ROM for Amlogic S9xx devices. slimBOXtv is based on official Android 9 or 10 firmwares but heavily debloated, tweaked, and optimized.
Technically, CoreELEC is Linux-based, not Android, but it is the most popular "ROM" for the S905X3. It boots entirely into Kodi.
Originally designed for the Ugoos X3 Cube but ported to generic S905X3 devices.