I don't think there is a solution by just changing the form of the python code (or at least it depends what your actual code wants to do, there is no general solution, and there maybe no solution to many cases). A technique I use since 2.x is to have the -b run import some_code; some_code.execute() and have a separate some_code python module (a bit clumsy but works in Windows). In Linux, you can stuff the python code, newlines and all, in a variable with a "heredoc" and use that variable with the -b
Code:
#! /bin/bash
# Read python code into a variable
# -d '' below necessary to keep the newlines.
read -r -d '' python <<'EOF'
gi.require_version('Gimp', '3.0')
from gi.repository import Gimp
print(f"Running batch in Gimp {Gimp.version()}") # Just shows that we imported Gimp correctly
if True:
print("This works!!!")
EOF
# Call Gimp passing the variable's contents in a `-b` argument
gimp -idf --batch-interpreter=python-fu-eval -b "$python" --quit