This is quick fix for sentry generated backup.json file.

sentry_backup_fix.py

import json

FILENAME = "backup.json"
FILENAME_FIXED = "backup_fixed.json"


def restore():
    with open(FILENAME, "r") as f:
        lines = f.readlines()
        lines = lines[lines.index('[\n'):]
    with open(FILENAME_FIXED, "w") as f:
        f.writelines(lines)
    with open(FILENAME_FIXED, "r") as f:
        data = json.load(f)
        for d in data:
            value = d.get("fields", {}).get("value")
            if value is not None and not isinstance(value, str):
                d["fields"]["value"] = str(value)
    with open(FILENAME_FIXED, "w") as f:
        f.write(json.dumps(data, indent=4, ensure_ascii=False))
    return data


if __name__ == "__main__":
    restore()

@Lucaber , my adapted script above should be of help if you have your original exported backup.

If i remember correctly, numbers were also saved incorrectly as base64 encoded strings with quotes. 8 -> "8" | base64.
This resulted in broken "sentry:option-epoch" options.