I also have a GX85 (the USA version of your GX80). Unfortunately, Shotcut is unlikely to give you a smooth editing experience directly on those files in native 4K (plus filters and multiple tracks) no matter how much hardware you throw at it. Shotcut is not designed to do processing in GPU the way DaVinci Resolve is. You can have a smooth enough experience if you put your 4K clips on a 1080p timeline. Or you might have a smooth enough experience if you first transcode your 4K clips to an intermediate format like DNxHR or ProRes and then drop them on a 4K timeline. If anyone else has found a hardware setup that is smooth at multi-track 4K, I’d love to hear about it!
This is pretty well covered in another recent post:
16 GB is sufficient for 4K work. I’m not sure Shotcut would even consume a full 64 GB. I would think 32 GB DDR4 would be your sweet spot so you can run other apps at the same time as Shotcut.
Any modern video card will support both.
@D_S keeps up with hardware trends and may have specific CPU recommendations. Here is a discussion thread from a year ago, with an Intel E5-2667 v2 being a great candidate for Shotcut work (maybe not so much for video gaming):