[Patrick Jones]"iMac 27" Late 2015 64gb RAM, 4 GHz Intel Core i7, AMD Radeon R9 M395X 4 GB - Internal Storage is the 3TB Fusion....I'm thinking about upgrading the Fusion to SSD and maybe changing out the Blade SSD to a 1TB. Not sure. Do you think this would speed things up to keep up with simple 100Mbps 4K, under 10min projects?"
A "simple" 100 mbps 4k project is H264, which is difficult for most hardware or software to handle with total smoothness. However it should not crash, hang or "quit". The solution is not modifying your Fusion Drive, and I'm not sure that's possible anyway. Our lead editor is using a 2013 i7 iMac 27 with a 3TB Fusion Drive and the FCPX library is 20 terabytes on a Thunderbolt array. It uses proxies but runs mostly OK on FCPX 10.3.4.
What version of FCPX and macOS are you running, is the media all on the internal drive, and how much free space exists on the internal drive? Are you editing the camera native files, or did you transcode to proxy or optimized media? If general you don't need optimized media but proxies can help with performance and editing smoothness.
If you have background rendering enabled in FCPX preferences, that can sometimes cause problems so try turning that off, then delete all render files in case any are corrupt or damaged.
By "quitting and not rendering" do you mean FCPX crashes or hangs, or CTRL+R won't render the timeline or it won't export?