I feel like we need to both group and separate.
We need to group because everyone across the spectrum has things in common, and a lot of the accommodations that "mild" and "severe" autistics need be similar (for example, needing quiet spaces), and we shouldn't dismiss these needs in those with "mild" autism because they're coping with things better than others. I feel like we also shouldn't dismiss how helpful the insight gained from autistics who are more easily able to communicate might be in determining appropriate care for those who are less able to communicate.
We need to separate because some autistics have needs that others don't. And it is of course important that we recognise that and give the more disabling impairments the extra attention they deserve.