You won’t be able to turn on ADP if you have Find My active on an older device - Apple will prompt you to remove the device (or upgrade it first) before you can activate ADP. I had to make a trip to my summer place to update an Apple TV to 16.2 before I could turn on ADP. (It’s a HomeKit hub, so I needed to do so, and I had to make the trip anyway.)
I was never able to get contact sync working on an older device not signed in to the Apple ID, despite Apple saying contacts were not affected by ADP.
But, again - ADP is optional. You can get by just fine without it.