Sorry I don't have a lot to offer in the way of help. I literally just installed the latest Mint last week, on a Phenom II with R9 380x and it installed all drivers automatically not sure if maybe the Threadripper is throwing things off for you? There are a couple of folks here with some significant experience with Linux. One I will mention now is colesdav . Mentioning him will make sure he sees this thread.
This forum is USER to USER and by far the expertise here is dominantly Windows.
The common advice here for Linux users here is for the purpose of expediting an answer I would suggest you also ask it in a forum dedicated to you Distro. Yours being Mint you could ask in a Mint and / or Ubuntu forum. The guys in those kinds of forums are usually very knowledgeable and worth asking. Not trying to send you away and Dave may be able to offer some help or another user might be able to help.
I know a lot of times I get one of the people in the Linux forums to tell me exactly what to put in my terminal and like magic it works.
This forum has been very helpful for me:
https://forums.linuxmint.com/
Good Luck!