The eGPU via TB3 is metered thus efficiency and speed will suffer, not to mention the fact that most of the video editing software is not really optimized to work with eGPU. Also, I have allot of weird stuff happening when using an eGPU , like multiple disconnects , system slow down etc .
They can take the power brick outside of the mac mini enclosure and make room for a decent graphic .
FinalCut Pro X, DaVinci Resolve and Adobe Premiere all provide ways for the user to specify which GPU to use for rendering. Apple does need to do a better job fixing those nagging bugs that still exist.
Apple is never going to put an external power supply on the current Mac mini form factor to put in a GPU. They ditched those thing in 2010 and they aren't looking back. Should Intel get off their fat ass and put 10th Gen iGPUs in their S-Series CPUs (Desktop) as they offered with Broadwell and Skylake and then simply let languish with UHD Graphics, then Apple would have a pretty decent solution for the Mac mini. After going all in on using desktop CPUs with the mini, I certainly don't want them to backtrack and go back to laptop (U- or H-Series) GPUs.