If you keep restoring your phone from backup, you are likely carrying something corrupt over with the backup. 32GB of System Data does seem a bit high.
Instead of restoring from backup, make sure you are syncing everything which can be synced to iCloud, the wipe your phone and set it up again as new. You can bring your Apps back over from the App Store as long of course as you are signing into the phone using the same Apple Account. And everything you synced to iCloud, such as messages, photos, passwords, etc., will download. But you will need to set up the phone all over, like Wifi, email, Bluetooth devices email, payment methods, etc.
For apps, you can easily find any app you've purchased all at once, by doing the following after you set up the phone again:
Tap the App Store App.
Tap the circle at the top right with your initials in it.
Tap Apps
Tap Not on the iPhone
Any app you've downloaded previously, but isn't currently on your phone will have a cloud icon next to it. Tap the cloud and it will redownload to your phone.
Final Note: Apple no longer sells any devices with only 64GB. It's just not enough anymore. The least amount you can get is 128GB, but even that to me is not enough space. The next time you buy a new iPhone, consider starting at at least 256GB or more.