Like most of the others, I can't help with the speed of rendering GFX100 files but I found myself in a similar position last summer. I had a 2013 Mac Pro with 1TB and 64GB RAM. It seemed to work but... I decided Apple Silicon, while certainly the future, was not quite ready for prime time so decided on a tricked out Intel iMac 2020, i9, 64GB RAM, 4TB SSD.
Actually, I believe the lossless compressed files on the GFX100 are actually smaller than the uncompressed RAW's from my Sony A7RIV. The uncompressed Sony files come into LR at 117MB and when sent to PS are 341MB. You could tell me if I'm incorrect. Many people have told m I'm crazy not to use compressed RAW with the Sony but I'm a slightly OCD detail freak and do not want to lose ANY information so have stuck with the uncompressed RAW. Storage is relatively cheap. The new Sony A1 apparently has lossless compressed RAW, a great feature, but I'm not about to drop $6,500 for it. The rest of the features are not relevant to what I do (eg 30 fps without blackout) and the A1 is still missing automated focus stacking (which is important to me).
Bottom line, I'd wait for the newer Apple Silicon machines to arrive. Let someone else be Apple's beta tester. I know one can wait forever but occasionally it does make sense.
Good luck.