First thing to note is that psi4 is python3 and your amber is python2.7, which won’t work regardless of PATHs.
AMBER should be python3 compatible already. Not sure how you can force the AMBER miniconda to the correct version. You could tell Ambertools to skip the miniconda and then manually install everything amber wants into the psi4 environment.
I am not sure I understand you correctly what of amber you have installed and what not.
Or did you mean “try to run amber serial or parallel” ?
There are possibly different solutions depending on your needs (serial only, MPI, GPU module, etc.).
The simplest is installing into your PSI4 environment the ambertools package from conda (https://anaconda.org/AmberMD/ambertools). That would be enough for QM/MM with psi4 in principle.