I think I get your question, but correct me if I've misunderstood.
We're migrating in several steps. Plone 5.1.7/py2 and Plone 5.2.5/py2 both use psycopg2 = 2.8.6 and relstorage = 3.4.5. Plone 5.2.5/py3 uses relstorage = 3.4.5 as well. I've tried psycopg2 2.8.6 up to 2.9.2 while troubleshooting this.
Process-wise, I have been using these steps as a guide: Flame alert: the Plone Python 3 upgrade story - #13 by jensens