I can’t answer your question, but what i can tell you is that the Android Runtime has nothing to do with (okay, is not closely related to) what version you use as a base for building or installing Sailfish OS.
People keep assuming this, so better keep spreading the information.
If you feel brave and want to reflash your phone, of course you can try… but if you want it to work and just get it done, trust the docs.
Why would you need to do stepwise upgrades with SFOS if you start from Android? Just grab the latest version and flash it.