So simplicity, let’s just refer to 5.9.0-RCs. FWIW, ParaView 5.8.1 built with Python2 does not depend on Python3 at all, so that requirement was coming from some other dependency either of ParaView or the simulation.
How are you building ParaView? Are you using superbuild? Are you directly building ParaView source? Are you using spack? What options did you turn on/off? Shared build? static build?
Is pvbatch or pvpython in your build working at all?