You're right - for whatever reason I had convinced myself YBCO had issues with both high current and high field. It's just high current - anything outside of a single grain has low current density.

Doing some more research to help make up for my spreading of incorrect info, I did stumble across this - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5472374/ - it seems like it's a variety of factors that make most HST not really commercially viable for MRI machines at current.

Mitsubishi has even made a small scale MRI machine with HTS - https://www.medicaldesignandoutsourcing.com/worlds-first-3-t... - though I can't find any newer details around it, despite the timeline provided in the article.