Google and MS don’t charge as much as Apple for storage, and you probably need you need to pay beyond the free limits, but it’s not a huge expense.
Once your installed Google Photos and One Drive on your iPhone, just tell the apps to sync all your photos all the time!
Now I appreciate that isn’t for everyone.
But it works, is reliable, and requires no technical knowledge of running your own service.
The other thing to do is setup a Mac that synchs all your iCloud data, One Drive documents and Google Drive.
Then back up that device with Backblaze.
This gets expensive as a Mac with decent levels of storage isn’t cheap!
I live in fear everyday or my primary Apple and Google accounts getting locked!
I’ve had accounts since day one of iTools and very shortly after Gmail launched….