Correct Thunderbolt Cable for 2012 MacBook to 2013 iMac

Elderly (88) neighbor inherited 2013 27" iMac and has been using a 2012 MacBook. We tried using the Migration Tool to transfer the MB data to the iMac and it took 6 hours. Yet we really didn't get much. His gmail was not picked up and when I tried to set it up in Mail, I got a notice that Google could not find a special file.

We tried it again and got the same results, pretty much. He is in a panic over his email problem. None of his passwords were saved, Firefox was not transferred over. If you've ever worked with such an older person they can be very cranky and, of course, they do not understand how data could be lost over the air.

We tried from his current TimeMachine back up and that gave us even fewer items. We're only going from Sierra to High Sierra, it can't be that big a leap.

So the next step is a direct machine-to-machine transfer. Having never used a Thunderbolt cable I don't know which cable to buy and the cable must be compatible with a late model 2013 27" iMac. I used up his entire day yesterday trying to get the transfer completed with partial results. When I told him he could re-build his contacts, etc., manually he was dismayed. His hand Arthritis is bad and I had to plug in a keyboard and mouse because the trackpad won't work anymore.

Personally I gave up on TimeMachine years ago and moved to Carbon Copy which clones the system. Thus, I always have an current external backup. I could suggest this option to him but CC is expensive and I know he will say no.