Cannot migrate data and settings between iOS 12.x and iOS 13.x

In the good old days when I could sync and backup using iTunes, it was possible to install apps and sync data and settings from iTunes directly to a device.

Now, I cannot do that. I can download apps on the device. I can restore, say, a backup made in Finder on an iPad running iOS 13.x on an iPhone X running iOS 13.x. I can migrate data and settings from an iPad running iOS 12.x to an iPad running iOS 13.x.

But I cannot restore data, settings, apps or a backup from an iOS 13.x device to an iOS 12.x or earlier device. Yes, I can manually -- and tediously -- move stuff "backwards" from a newer device/iOS to an older device/iOS. And if I cannot move it, I have to recreate it or reinstall it.

I discovered this today when I tried to duplicate the contents of my iPad Mini 5 running iOS 13.6.1 onto an iPad Mini 3 running iOS 12.4.8. No go. The best I could do was to use the camera of the Mini 5 to capture the blue "cloud" on the Mini 3 to transfer basic settings. Then I had to download all the apps from the App Store -- at least, the ones still alive and compatible with iOS 12. I spent a lot of time downloading the 7,000-some music files via Finder. But I couldn't download photos since I don't store mine in iCloud.

I understand there are differences between iOS/iPadOS, etc. versions that cause problems. It used to be fairly easy to duplicate the contents of one iOS device to another, even when they had different versions. Now, Apple has made used to be easy very difficult to impossible to achieve.

Is there a way to easily "dupe" one Apple mobile device to another if they run different iOS, etc. versions? Or am I sliding quickly into Luddite-hood?