Cool idea!
IMO one important consideration would be the oupt sequence length. AFAIU for BART’s denoiseing objective the output sequence length is same as the input length,where as for T5 it’s quite small, which would lead to faster training.
adding rotary embeddings also seems like a good idea!
Also what do you think about  deep encoder-shallow decoder 3, looking at the ByT5 paper it seems it’s worth exploring.