Sideboard guide:

There are linear aggro decks (RDW, Knights). Against them you want to:
-2 or 3 Trails of Crumbs (because we don't have the time to use it but 1 or 2 might be okay if the game drags out)
-2 Midnight Reapers (they are killing us fast enough already, no need to help them)
-2 Karn (we don't have the time to fetch things and there is too much pressure to win with Citadel)
+3 Scorching Dragonfires (we really don't want to die)
+2 Act of Treason
+ Stonecoil Serpent

You might also want to bring in Call of the Death-dweller (which currently is a 1-off in my sideboard in place of one Midnight Reaper) or Golos (could help slow them down but it dies to Shieldbreaker that they are definitely boarding in).

There are Lurrus and Obosh decks. Against them you want to:
-2 Trails (priority #1 is slowing them down so we sacrifice card draw for removal)
-2 Midnight Reapers (same as against the aggressive decks with the added note that you really don't want to have it Claimed)
+3 Dragonfires (Priest is public enemy #1 with Lurrus himself coming as a close #2)
+Call of the Death-dweller (or +Golos if you don't run Call)

There are Fires+Keruga decks. Against them you want to:
-Trail (too slow, the game is decided on turn6)
-4 Claims (the only target is Bonecrusher)
+Midnight Reaper (in this matchup a creature based draw engine is better because it gets to apply pressure)
+Call of the Death-dweller (Devil with deathtouch will decimate their board, they can probably deal with it but by that point we will be able to pull the Citadel, hopefully) (or 2nd Reaper if you don't run Call)
+3 Act of Treason (this is one of THE matchups it is here for)

There are Yorion Elemental decks. Against them you want to:
+3 Act of Treason
- anything you want (I generally cut an Oven, a Reaper and a Goose but you can tweak the numbers on the Geese, Ovens, Trails, Reapers and Cats however you want)

Then there are Yorion Control decks:
-4 Claims (no targets other than Shark tokens and Walls)
+Reaper
+Call
+Spyglass
+Serpent

That pretty much leaves the Temur Reclamation. Against them you want to curl up and cry. If you can pull the Citadel on turn 6 and it actually resolves you will win. If you curve out and use Priest every turn while they don't have any answers (probably game1) you will win. Other than that you pray they get manascrewed or flooded. No sideboarding will help here.