When I’m running these not as raids, the only way I know of to exit the dungeons is to run back to the beginning.
Is there no portal or something to take you out of the raid once you finish? I could hearth home, but I’d still have to come back and do the last one.
Usually no; the designers assumed that at the end of the raid everyone’d hearth or portal out. 
(this is why I started with all the LFR, but I have more alts than you do, sounds like, even if most are at 110 still)
There sadly aren’t that I’m aware of. Jade Serpent fortunately ends you right back by the entrance, but the majority of the instances require you to run back or hearth.
If you have at least 1 toy that acts as a hearthstone, you can delete your hearthstone and use the stuck feature on the help window to teleport to the last graveyard. Usually much closer to the start of the instance and you dont lose any durability as well.
What you could do, if you wanted, is after you get to Shrine of Two Moons/Shrine of Seven Stars, set your hearth to there. You can always get back to SW or Org from their, those portals are still up.