If it's local, I wouldn't make it tiered by library size.
I think you're being perhaps optimistic with your $15/mo tier. I'd love to be wrong, but that money would pay for a new 10 TB HD every year. Maybe other people won't think that way? Are you using s3? Your costs might be lower using glacier or backblaze.
You should also take a look at https://github.com/mholt/timeliner. Open source, easy to install, runs locally for free. Worked perfectly on my very large library.
However, both your service and timeliner have to deal with Google Photos and the file mangling they do. Most tags are deleted, and several are modified (!!?) when you fetch them via takeouts or timeliner. I don't think there's anything you can do about it, because you're downstream of where the damage happens.
Good luck! You should try PhotoStructure, BTW. Link in my profile.