OpenBoard is no longer maintained. Heliboard is a good alternative.
I think Google Play on Graphene is a good compromise, since at least it does not have root access. Unfortunately it is very crippling to completely avoid Google on Android.
That being said, I heard others are using work profiles to isolate all apps using Google Play from their more private apps. I’m not doing that, but work profiles are nice for … well work apps.
https://github.com/openboard-team/openboard
Last commit is two years ago. I’d recommend checking what you have installed. A quick search on GPlay finds what appears to be a clone that may or may not leak user data.