As Phalos said, Federal Appeal is the next step. I think it would be beneficial in your case to speak to an immigration lawyer to see how strong your case is. The lawyer can look at your case notes and see why you were refused. Also you should be able to provide a reason why you waited from refusal in 2018 to now to proceed. Hopefully you have more evidence now in addition to what was submitted originally to help your case along. In any event, I think speaking with an immigration lawyer may give you better direction.
Perhaps anyone here in the forum could also provide some guidance and positive support or this couple.
I wish you all the best - stay strong - and keep fighting.