Excellent planning.
To add to your collection of backups, you might consider Google Backup and Sync, which has for awhile offered unlimited photo storage at “High quality”. This may look a bit worse than original if highly enlarged.
See photos & videos with Google Photos & Google Drive may have a note at the very top about a July 10 change to separate Google Photos from Google Drive a bit more, but I take that as a sign that the note at bottom about “High quality” being exempt from quota (and a base quota is free) will stay after the change.
I’m not sure if that means your usage pushed you into a higher storage or transfer size, but there are still some backup services that are flat-rate (though there are fewer than there once were). One that explains clearly why they do this is Backblaze Personal Backup. They compare against competitors on this page, which might give you an idea of how much consumer-grade commercial backup costs (not all that much depending on the situation). Backblaze has a non-flat-rate service called B2 for its more ambitious users, such as those would that run Duplicati. Their own client is aimed at simplicity – not good for power users.
We’re The Backblaze Cloud Team (Managing 750+ Petabytes of Cloud Storage) - Back 7 Years Later - Asks Us Anything! explains their rationale for keeping unlimited for users – it’s part of an ease-of-use goal. They also talk about how being able to support huge backups (Duplicati doesn’t do this well) helps others.
How Backblaze Got Started: The Problem, The Solution, and the Stuff In-Between explains initial strategy, then later on they added the B2 service that Duplicati and other more sophisticated backup situations use.
Setting up with Dropbox simultaneously commented on your technical limitations and solved a hard issue, so I’m not sure whether you prefer to go for “simple” or not, but Duplicati can definitely get far from simple because you’re assembling it yourself (“bring your own storage”), then one faces issues like the one here. Having one service that supplies the client and the storage has less integration work, but is less flexible…
Different users and uses.