For the Zereth Mortis stuff, you can ship all of that to an alt until you need it. Hell, you can do that for a lot of things you’ll only eventually need at a specific point. That alone will probably free up like 15-20 slots.
Also be sure to make use of Void Storage, if you have any Legion Legendaries kicking around in there. And any other mementos that aren’t labeled stackable or “unique.”