I think we can all agree that adultery is grave matter. I think where the wiggle room might be is on the issue of culpability (remembering the three conditions for a sin to be mortal). ** It could very well be that the aggrieved party in a divorce that re-married outside the Church, has remained faithful to the new spouse for many years, and has a family with that spouse, might be seen as having subjectively diminished culpability for an objectively grave sin**.
I’m thinking the synod might lean in that direction: study each individual case pastorally, and determine the degree of culpability. Objectively, it is adultery. Subjectively, being (albeit invalidly) married and faithful to that spouse for many years, certainly isn’t same as, say, a married man on a business trip looking to hookup with someone in a bar.
Or it may not go that way at all. It may be as was said, a streamlined annulment process. Who knows?

We have to trust that the Magisterium will remain faithful to Christ’s teachings in a way that will consider the pastoral care of a very large and growing demographic. So I prefer “just in time worrying”. Why worry about something that hasn’t even happened yet?