What Options Are There for Importing Mini DV and Hi-8 Tapes to Thunderbolt?
Well for MiniDV, you want to find yourself should budget permit, the Canopus ADVC Conversion Unit. It is by far the best realtime hardware (external small box about the size of a VHS tape), that will allow you to on the fly input in HDMI, FW-400/800 and component video and output over HDMI or FW.  (So it's flexible). 

The encoder is handsdown leaps and bounds better than any of the cheap capture cards, and even the BlackMagic pocket adapters. 

Hi8 Tape is Analog, unless it's Digital8. In which case you'll need to capture via Analog, and output via HDMI into your Mac. If your Mac supports HDMI capture, that will work, or you can buy an inexpensive BlackMagic h264 ProCoder to handle the input. 

You will still need source decks. The Canopus box I mentioned, (which is no longer made) you might find on eBay (I found my 2nd one there). It also supports HDV, the high-definition mini-dv format, with virtually zero loss in quality (visually). 

Other than that- you are going to have to set up a DeckLink card (really the only "better cheapest easiest" option in terms of a one card fits all. 

You might be able to get a DeckLink card which handles all the input types you need (they have a lot of models, which differ in what they allow in/out). I'm guessing around $1500. 

Then connect your decks, and capture. 

As for Thunderbolt. I am sure the newest DeckLink cards support it (I would think), but will be very pricey if they even do. 

My advice, either find the Canopus box I mentioned, or get a H264 ProCoder by BlackMagic; pair it with the appropriate BlackMagic converters ($300 each) to get your deck to feed to the ProCoder (Which only takes HD-SDI in), and then capture over USB 3.0. EDIT:  The newest ProCoder may accept HDMI in/out... I'd look at BM's website to see what their current lineup offers. 

Hope this helps.