Thanks Steve…always good to know I had a clue 
Part of the problem I saw in these failed distortion corrections was that it looked like a little tornado had touched down in the middle of the brain. Regardless of whether the outlines of the brain were smeared, the tornado bothered me…and a mask would not capture that problem!
My initial fmriprep v20.2.7 LTS results came back and they’re looking much better in terms of distortion correction than what I got for my v23 syn-sdc and sdc.
Thanks for the suggestion. I tried v20.2.7 LTS with fieldmap correction and the results still look awful ; (