My move from iOS years ago to grapheneos was primarily about taking back control of my data (away from cloud services). I decided to leverage what I already had at home, which was a NAS device. What I ended up with is the following, which solved your use case:
- each person in my house has a user account on the NAS
- the NAS is not cloud connected nor open to the internet
- each user downloads a drive sync app directly from the NAS vendors website
- I have my GOS phone, Tablet, Linux laptop and Mac desktop all syncing with the NAS.
- I have different sync folders and sync types setup. For example, I have a folder for called PasswordDB and my kdbx file is stored there, the folder is set for two way sync and offline use (a local copy is stored on every device). KeepassX on my Mac, KeepassDX on my GOS devices, and KeePassXC on my Linux boxes all sync this same folder.
- for media, I have a Pictures folder, Videos folder. Each folder has a subfolder named for each device, the folders are setup for one-way sync (device to server) and not for offline use.
This type of approach works great across device platforms and keeps you in control of your most sensitive data (password db and pictures). For other, non sensitive, data objects, you could use cloud services.
For redundancy, a replica NAS is an option, or you can setup an encrypted cloud sync with box or dropbox where you create an encryption key, and the objects on your NAS and encrypted on the device, then synced to the cloud service so the service cannot decrypt them. This is usually a paid capability due to the amount of data synced from the NAS. The benefit is you can choose which folders to perform an encrypted sync with. If your NAS is irrecoverable for some reason, you can setup a replacement, get the sync credentials and encryption key from your keepass db (which should be accessible since this architecture keeps a copy saved across your devices) to repopulate. An alternative is buy a second NAS, configure it as a mirror, perform the initial sync then move it to a friends or family members house and sync it over a VPN.