Motion 5: 360 Graphics, Export hits "infinite snag"

I'm trying to create some 360 motion graphics for use in a 360 spin video. I can view them in preview, but when I change the position of the graphic such that some or half of the graphics are "behind" the 360 center camera, with a Z value of say 4000-5000, it will export about 23 seconds, then suddenly go to essentially infinite export time estimate, and it stays there even if I leave it for say 6-8 hours. Any thoughts as to why this might happen? I'm on a 2017 iMac with 64GB RAM, 3.6 GHz Quad-Core Intel Core i7. Tried both the built in graphics processor and a Radeon RX 570 eGPU with 8GB RAM. Also have tried both on OS X Sierra and Catalina. Emitters were adapted from a 2D file, pasted from that file to a 360 file.