Regarding the first two points, maybe Kopia [0] come close. It has both GUI and a CLI. For the GUI, it saves your backup key for you (although I have to admit I didn't check how much securely stored it is), but you still have to keep a copy yourself in a password manager or similar in case you need to access your backup from some other machine. AFAIK, for the CLI you are completely on your own regarding secrets management. But it's also true that the average user doesn't have servers to backup, so the GUI would be fine.
Regarding the management of storage, what do you mean? Kopia already supports different "storage backends" out of the box which can be configured via the GUI. Do you mean that you would like it to be able to merge the storage of different storage providers, so that you can use multiple free tiers to get the storage space you need?
Regarding the management of storage, what do you mean? Kopia already supports different "storage backends" out of the box which can be configured via the GUI. Do you mean that you would like it to be able to merge the storage of different storage providers, so that you can use multiple free tiers to get the storage space you need?
[0] https://kopia.io/