Very slow RAM preview

RAM usage in AE is seriously overstated and 64 GB is plenty. Yes, there's always a minimum that will be required for AE to breateh, but you're far from suffocating it, either. And lest we forget: puget Systems still sell computers, even if I wouldn't attest them shady motives. 😉 Your excessive CPU usage furthers my suspicions that this is a leak/ bug in the H.265 processing, so I can only repeat my advice: Try to transcode  at least a short clip for testing purposes in Handbrake or whatever tool may have come with your camera and see if it makes a difference.

Mylenium

Without any actual info about your system, cache settings, preview settings, options for hardware acceleration and multiframe rendering and all that jazz we can't really tell you much. To me this simply sounds like a leaky graphics driver messing thinsg up and/ or the footage being encoded in an unusual/ odd way so AE struggles with decoding it. At least in the latter case you can try to transcode it in Handbrake and similar tools just like of course you can update your graphics driver and all the relevant settings I mentioned. For anything else more specific info is required.

Mylenium