Thanks for the assistance. I have an A7IIR sony mirrorless (and a few olympus mirrorless too plus several various lenses from old cameras), but not so much video. These days if I'm holiday, I mostly use the iPhone Pro. At least I get something! I wanted the A7IIr's low light for campsites and such, but its extra lense weight makes me miss micro four thirds. I sold my best my best micro four thirds lenses but kept the cameras ... silly me?
I had thought I could compress with my MacBook pro circa 2017, with its quad i7, and it has a seperate GPU. But its security chip is the T-1, which compresses no better than not having one!! Still, I will do it video by video, so compression speed can be done at nighttime. I've even looked at a used 2018 mini - but I'd rather spend on the 5,1 because its storage capacity seems a better price/performance.
For me it's not at all Pro environment. Also I have CS6 photoshop and that stops working with Catalina - and I hate the idea of renting software. The liberation of PCs was getting away from rental software. Now MS and Adobe and all the rest - they've taken over their competition, so now they all want to bring back rental software. Like making phones that even the most fearful greenies still buy a new one every two years.
I'll likely go the rx580 route, and keep the 2nd 16 path slot for a raid card, boot of a card in that, put in another to see how it goes, and keep a raid for later. Keep another single mechanical inside for backup. Not sure what i'll do with the finished videos - one issue with those, is how long will their formats stay current, and where does one store them for the long term?