I've got the same exact issues OS 10.15.6 MacBookPro 15", 2.4 Ghz, 8-Core i9, 32GB 2400 DDR4, 1GB SSD, Radeon Pro Vega 20 4GB, Intel UHD 630 1536MB, FCPX 10.4.9.
It reminds me of the old days, when we had to click of the Background Render to work. Yes I do have Coremelt, and Motion VFX and Tuxera, and some more...but the only thing that changed was FCPX, so its should be something fixable in FCPX. I've got patience, but my clients don't.
The media is all 4K 25p Rec. 709 from a SONY a73....
....working in a 4K 25p timeline, with about 100GB of footage on an external 2TB Sandisk Extreme Portable SSD.
....450 GB free on my macHD and 1 TB free on my external Sandisk.
If any of you have had any solutions with the reformats or any other solutions let me know... thanks... APPLE PLEASE UPDATE!!!!!!