This is correct. You will want to divide each element of the SliceTiming array by the RepetitionTime. ChatGPT could help you create a simple Python script to convert a BIDS JSON sidecar into a fsl slicetimer format ingle-column text file and then run slicetimer for you. This is a bit out of scope for dcm2niix, so you can also ask the FSL jiscmail group for FSL specific questions.