I’m editing on MBPRO of similar specs and age w 4K regularly.
One difference is I transcode all my camera clips to ProRes and that becomes my media.
Reason being I find the intraframe format gives me smooth playback and fast operation with PPRO. I’m a long time Final Cut user from the pre-X days so the transcode is no issue to me and these days doesn’t take too long.
All my media is accessed from- and video previews are rendered to- external RAIDs (SSD and not) via Thunderbolt and USB 3. I use both RAID 0 and 5 depending on complexity of project.
Suggest you update to latest PPRO and try a RAID if you haven’t yet.