Force restart iPhone 8 Plus
Press and hold both the Volume Down button and the Sleep/Wake button at the same time. Continue to hold those buttons until the Apple logo appears, release both buttons. (DO NOT release Side Button when invited to Slide Power OFF).
Once the above is done then restart the data transfer process again.
If the above does not work then Do you have a backup created either on iCloud or on your computer?
Method 1 - Backup your old iPhone on your computer like this 👉 How to back up your iPhone, iPad, and iPod touch - Apple Support and then Restore your new iPhone from that backup like this 👉Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Axel F.