What is the best Thunderbolt adapter or cable to connect a 2TB Time Capsule to a 2019 27" iMac for data migration

The USB port on a Time Capsule is for attaching a peripheral to be shared by computers via WiFi. It is normally used for a printer or disk drive. The USB port cannot be used to connect a computer to the Time Capsule.

You need to use WiFi or Ethernet.

You do not need a Thunderbolt adapter.

The iMac and both 2T Time Capsule models have USB ports, so all you need is a standard USB cable.

Yes, USB is faster and more reliable than using Ethernet.