The easiest, most reliable way to migrate:
- Make a current backup of your old Mac on an external drive. This can be a Time Machine backup, or a bootable clone backup ("startup disk") made with Carbon Copy Cloner or SuperDuper!.
- Transfer the drive to your new Mac and point Migration Assistant at it.
This avoids the potential unreliability and slowness of migrating over Wi-Fi.
Another possibility might be to get a Thunderbolt 3 cable, use it to connect the two Macs, and migrate over a wired connection. But those cables are expensive, and you've probably already got a backup drive on hand.
Transfer to a new Mac with Migration Assistant - Apple Support