Quick start phone data transfer for iPhone 11: errors, bugs,

Hi,

I've spent the last 6 hours trying to migrate data from an iPhone 7 to iPhone 11, using Quick Start --> direct phone transfer.

1. First, data transfer always failed at the beginning, with the message "Unable to transfer data", so I updated my old phone to iOS 13.5.1 (from 12.3)
2. Then, it still failed, and Apple support told me it's because the new iPhone has iOS 13.5 (and not 13.5.1), so I needed to update it through iTunes -- as both phones needed to have the exact same version...
3. I connected new phone to iTunes, but the latter showed "Error 4000 - cannot connect to phone" !!
4. After research, I updated iTunes, and even Windows, restarted, connected the new iPhone, and then connection worked!
5. I installed the new iOS 13.5.1 via iTunes, then restarted new iPhone
6. I launched the Quick start process again between both phones, and this time, data transfer finally started !
7. However... when transfer reached its end 1 hour later, I received again the same old error page "Unable to transfer data" ! and the only button I can press on the screen is "Reset my phone" !!

Resetting phone means also bringing it back to iOS 13.5, which means that at every failed loop, I'll need to spend an hour upgrading phone firmware using iTunes, and another hour trying to transfer data between phones.

This is quite painful... These preliminary conditions are not clearly mentioned in Apple's support page, and I can't find solution to this infinite 2-hour loop of transfer failure.

Please help.