Strange that Google pulled the older versions for the 6a but indeed as you report they are not available: Full OTA Images for Nexus and Pixel Devices | Google Play services | Google for Developers
I know I flashed stock A15 on a 6a in the past 6 months.
It is not exactly clear what caused what, but several users either with a non-stock install (maybe Calyx, maybe Graphene) but also some installing on a stock A16 base would report various possible issues:
- Inability to enable Developer Options without it crashing was a common clue
- Various permissions for other things or unbootable devices going to a black screen, etc. (sorry didn’t take the time to review them in specific right now)
But if you do not have any issues and you are happy then no problem to stay with what you have.
iodé 7 is set to release very soon. If you want to go to the beta now, the best is to install the 6v3 version of the Updater app that would expose the v7 beta and allow it to be installed OTA as described in the iodéOS 7 (beta) is here! post.