Kurdish groups have been forced by Trump to side up with Assad. From another angle, Assad lacks capacity to control North Syria without the Kurds and so must cede to a federative governance style. The Kurds have conservative Sunni tribes on-side and many of that number won't deal (directly) with Assad. The Kurds need to be the go-between. And layered on top of that is a misbehaving Iran.
What I have pushed for years now is a Sryian only discussion table (with no war lords invited). The first thing they need to discuss is a roadmap to cement bread an butter stuff and to rebuild trust. Any constitution at the moment must accommodate a high level of regional government rule if the solution or interim solution is to be tolerated by all parties. I fall short of saying accepted by all parties.
Assad needs to get it through his head that is not going to be a dear leader with every minor decree floating though his office for signature. I note the view that if he so much as blinks he will be eaten but Assad does not have to blink if he takes a proactive approach to a Russian and US supported project. Continuance of Assad;s tired old bluster will only lead to annexation of further Syrian territory by the Mad Sultan and extended Iranian influence in what is left of the country.