> the only cloud backup solution Apple allows you to use
Not quite. You can still have automatic local backups set up for iOS and macOS devices to your own NAS. And that NAS can then do cloud backups of whatever is on it in any way you want. It's certainly more effort than the stock iCloud solution, but it's still an option.
OK, if you have or buy a $599+ Mac from Apple in addition to your iOS device, and first connect your iOS device with a USB cable, and then enable the optional Wi-Fi sync, and regularly connect the Mac and the iOS device to the same Wi-Fi network while the Mac is not sleeping, and configure an e2ee cloud backup on the Mac to include the iOS backup, then that is actually a way to achieve a third-party e2ee cloud backup. Though it's stretching the definition a bit due to the requirement to have the Mac connected to the same Wi-Fi for the backup to occur, I'd consider a true cloud backup solution to work on any network connection or even cellular.
I'm willing to bet that the number of people who have ever set all of that up as described is in the triple digits worldwide. A rounding error.
I'm pretty sure your estimate is wrong by at least several orders of magnitude. Lots of iPhone users also own MacBooks, for starters, and iPhone will nudge you to set backup when you connect it to any Mac. And of course they are going to be on the same Wi-Fi network most of the time, too, when that person is at home. I'm not sure how sleep plays into it exactly, but MacBooks do wake up on their own to do their backups and things like app updates, so I wouldn't be surprised if an iPhone can actually wake up a sleeping MacBook to back up to it.
So the only case that is relatively unusual is having the Mac back up to a local NAS, but that's only because NAS themselves are a power user thing. Still, turnkey ones like Synology etc are much more common than "triple digits worldwide", and if you have a Mac and a NAS, why wouldn't you set up Time Machine to backup to said NAS?
via USB (or possibly local Wi-Fi) and your computer.
iTunes (or Mac OS's built-in iPhone sync) is the recommended way to do this, although the protocol has been reverse-engineered to hell and back and third-party software exists for it. iMazing is the most notable one, although there are probably others, and you could hack something on top of libimobiledevice if you really wanted to.
Getting those backups from your computer to the NAS is an exercise for the reader.
Not quite. You can still have automatic local backups set up for iOS and macOS devices to your own NAS. And that NAS can then do cloud backups of whatever is on it in any way you want. It's certainly more effort than the stock iCloud solution, but it's still an option.