As someone who values the essence of GOS and all its components, I am aware that decrypting outside of Seedvault is not within the realm of possibility.
I understand your frustration with the unreliable nature of Seedvault and the potential loss of your important data. It can be disheartening to give trust in a backup system and have it fail us.
So for your backup, it may be worth considering different methods of backing up your data in the future. While Seedvault might work for some, for me it has not been a reliable option. Perhaps explore different backup options such as manually exporting your data.