Would Truenas fit as immutable? I guess it doesn’t stop you from changing things, but doing so might break the next update.
Configuration can be exported. Disaster resolution of fresh install and restore configuration has worked for me. No data loss and even the Virtual Machines started right back up.
There was a payment app (maybe wallet?). Then they killed it and forced everyone to Pay. It was terrible, and everyone hated it. They gave up on that and brought back Wallet. They are now ending Pay. This is a good thing.