I certainly wish I was here to report an answer, but I am here because I have the same issue with both of my Thunderbolt 3 ports on my 2017 iMac 27". Upgraded the hard drive to SSD. The drive is so much faster than the fusion thing, but both thunderbolt 3 ports no longer work. I was using one of them to connect to an external monitor just fine before the upgrade. I called the guy whom did the work and he got pissy with me. I've tried everything I can try.