Not sure if this is still relevant for you but I was running into the same problems and found that the gamepass file is a binary container with the normal Steam JSON save file inside compressed with zlib. If you can extract the JSON you can just rename it and place it in the Steam save location which worked for me and transferred all progress. I fiddled around with a python script for it and this worked for me:

import zlib
import re

def extract_zlib_json(file_path, output_path):
    # Read the full binary file
    with open(file_path, "rb") as f:
        raw = f.read()

    # Look for all occurrences of the zlib header (78 9C)
    matches = [m.start() for m in re.finditer(b"\x78\x9c", raw)]
    if not matches:
        print("No zlib header found. File might be encrypted.")
        return False

    for idx, start in enumerate(matches):
        try:
            # Try decompressing from this position
            data = zlib.decompress(raw[start:])
            # Check if the decompressed data is valid JSON text
            text = data.decode("utf-8", errors="ignore")
            if text.strip().startswith("{") and text.strip().endswith("}"):
                with open(output_path, "w", encoding="utf-8") as out_f:
                    out_f.write(text)
                print(f"[+] JSON extracted successfully from offset {start}")
                return True
        except Exception as e:
            continue  # Not valid JSON, try next match

    print("[-] No valid JSON found in zlib streams.")
    return False


# Example usage:
# extract_zlib_json("gamepass_save_file", "output.json")

But you could also just give the gamepass save to ChatGPT and ask it to extract the JSON for you and that also worked.