When `plutil --convert json` fails due to "invalid object in plist for destination format", use Python to serialize everything except for the binary data. Based on a Stack Overflow answer.

plist2json.py

#! /usr/bin/env python3

"""Read Apple Plist files and output JSON data."""
# https://stackoverflow.com/a/72958435/5201675

import argparse as ap
import json
import plistlib
import sys
from typing import IO, Any


def main():
    """Run the main script logic."""
    parser = ap.ArgumentParser(description=__doc__)
    parser.add_argument("files", metavar="FILE", nargs="+", type=ap.FileType("rb"))
    args = parser.parse_args()
    for file in args.files:
        print(plist2json(file))


def plist2json(file: IO[bytes]) -> str:
    """Parse a plist string and return a JSON string."""

    def not_jsonable(_object: Any) -> str:
        return "<not serializable>"

    return json.dumps(plistlib.load(file), default=not_jsonable)


if __name__ == "__main__":
    sys.exit(main())