I guess part of the issue is that I have my iPhone, where I don't want to lose photos/videos (hence the iCloud subscription, where it will all sync to the cloud) and a laptop (hence the Carbonite for what's there, and Google Drive for what's not). So maybe this is not too much I'm paying for?
First, it's important to understand that cloud drives like Google Drive or iCloud or OneDrive or Nextcloud aren't data backups in the traditional sense, since they're designed to asynchronously update with any and all file changes, so if it's deleted or corrupted or changed in one location, it's gone or corrupted or changed everywhere. They're more like network drives in that regard. This isn't to say they don't usually have file revision history and a temporary recycle bin to recover from, but they aren't
backups in the traditional sense.
Actual backups, like what Carbonite or Backblaze or CrashPlan do aren't asynchronously updated. They take snapshots of directories and files at specific times for archival and catastrophic data recovery purposes. This also isn't to say that you can't potentially use cloud drives for actual backup storage, but that's besides the point and a bit beyond scope.
The iCloud prices aren't terrible for async live cloud storage, honestly, but unless you're regularly storing more than 1.2TB of data in backup archives, the Carbonite prices are... and of course there's always the privacy concerns with Google.
Here, have a friendly overview of how to set up Duplicati to back up to Backblaze B2 to give you an idea of how easy it would be to ditch Carbonite. The instructions are platform agnostic, because Duplicati is a tiny little server that runs on any OS that you can change settings on through your web browser, and free software.
https://www.backblaze.com/blog/duplicati-backups-cloud-storage/The nice thing about Backblaze B2 is you only pay for what you actually use instead of being charged a hefty fee for a huge promise of storage space you'll likely never fill up.
It does sound like you're running multiple platforms, though, given the divide between Google Drive and iCloud. Nextcloud has clients for all platforms, and does auto-syncing of stuff like photos on your iPhone.
CloudAmo has hosted, managed Nextcloud servers with admin access starting at $3/month for 100GB, which is half the storage for the same price as iCloud, but you gain the benefit of having a client for Mac, Windows, Linux, iOS, Android, weird stuff like Windows Phone... and you can set up accounts for family to use it, too, and directly share and sync all that with each other as well.
However, learning a little bit about Nextcloud before romping around in an administrator account would be advised... otherwise, stand alone Nextcloud accounts if you want to pay might be more your speed, as you have all the advantages of cross-platform support like with Dropbox and Box, but cheaper because it isn't proprietary. SpryServers aren't as cheap for this, but their shared accounts are a bit less oversold than CloudAmo's stand alone accounts, but CloudAmo's prices on those stand-alone shared accounts are free for the first 3GB, and ~3¢/GB/month above that, with a ~10% discount on annual payments.
Not saying Nextcloud might be a perfect fit for you, or even the cheapest option, just that it
is an option that's frequently cheaper than most other async cross-platform network storage services with a lot less datamining.