Welcome @acvdh to Neurostars! Thank you for your interesting question!
Regarding the behavior of FMRIPREP with respect to the use of the fildmap-less option, I encourage you to read this post: Using --use-syn-sdc with FieldMap Data - #2 by effigies?
I am not totally sure but I would guess that a correct value of the TotalReadoutTime is not strictly necessary when doing SDC with the field map-less approach. You can check the details of the field map-less implementation in the paper cited here :Question about Fieldmap-less distortion in QSIPrep