It's possible to use at least some TB3 eGPUs with TB2 and adapters, it's not officially supported by Apple on macOS though. They are still insanely expensive in

It's possible to use at least some TB3 eGPUs with TB2 and adapters, it's not officially supported by Apple on macOS though. They are still insanely expensive in my opinion for what's basically a case, small PSU and PCI-E to TB3. The cheapest ones that can take most cards are around $300 and usually have some downside. I considered it for my Mac Mini but in the end it just made more sense to sell the Mini and buy a desktop PC with space for a real GPU, more storage, faster processor, etc. I mean my case, power supply and motherboard was less than the price of a Razer Core X...