Following your steps it worked for me:
iPhone 6, iOS 11.0.2, iPad Pro 11.0.2, iTunes 12.7 on MacBook Pro with 10.12.6
1. Opened iTunes 12.7, connected iPhone/iPad via usb (wifi connect works as well), clicked on iPhone device icon
2. Opened Finder, navigated to Music/iTunes/iTunes Media/Tones folder (ringtone *.mr4 files are still there)
3. Selected all files, right clicked to then copy all
4. Clicked on Tones under On My Device

5. Clicked on Edit in the iTunes Menu bar, then clicked on Paste

6. All of my *.mr4 files (purchased and custom) were copied to my device
7. On my device, Settings > Sounds > Ringtone, all of my Ringtones were available to assign to my contacts
8. Ringtones copied this way also found under Settings > Sounds > Text Tone