Firewire 400 to Thunderbolt 3

In order to use it with a 2018 Mini, there is going to be

a long string of dongles. Starting at the Mini you will

need a Thunderbolt 3 to Thunderbolt adapter.

https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter?fnode=8b

You will then need to plug in your Thunderbolt to Firewire

adapter into that.

Then you need a 400-800 adapter similar to this:

https://www.amazon.com/elago-FireWire-Adapter-MacBook-Computers/dp/B003L4P872/ref=sr_1_4?dchild=1&keywords=firewire+400+to+800+adapter&qid=1585830507&sr=8-4

Or, you could update your camera to one that uses SD cards instead of tape.

Just the other day I tested my old Sony TRV320E with my Mac mini 2018, Mojave and iMovie HD 6.0.3:

Mac mini 2018 - Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter - Apple Thunderbolt to FireWire Adapter - FireWire 800 9-pin to FireWire 400 4-pin cable - Sony TRV320E D8 camcorder (essentially miniDV with Hi8 tape).

I could import/export DV just fine except occasionally I had to disconnect/reconnect the cables or restart iMovie 6 HD because the Firewire connections seemed to freeze.

iMovie 6 HD can be installed to Mojave via Pacifist, and then updated to 6.0.3 (or maybe install 6.0.4) one of its package file must then be edited with a text editor so it runs on Mojave. Get Info > Run in low resolution fixes the freezed iMovie video display on import (the import is OK even without this fix).

...but in the end I used my old 2004 PowerBook G4 on Mac OS X 10.5.8 to convert old analog VHS-C tapes to .dv and used the new Mac mini 2018 to deinterlace & convert it to H.264 with MPEG Streamclip (the old PowerBook was way too slow for that encoding).

I have used those adapters also to connect my old Firewire HD docks to the Mac mini 2018 but eventually bought an OWC dual drive USB-C dock to reduce cable clutter. The Thunderbolt LG display also had a few second blackouts, possibly caused by the adapters.