As someone who runs a deck very similar to this, I'm just looking for some clarification on your pokemon:
Do you run M Aggron EX? It's in the title, but not on your list.
Is the Excadrill you run the ancient trait one or the regular rare one?
I can 100% assure you, you will NOT require 4 Professor's Letters.
Current recommendations based on your list:
-1 Battle Reporter (really unreliable supporter, especially against opponents that play very aggressively)
-1 Caitlin (interesting card, but just not consistent enough to warrant being your supporter for the turn)
-2 Professor's Letter (take it from me, you don't need 4 in this deck)
-2 Pokemon Centre Lady (from playing other decks that run PCL, one is plenty for healing off)
-1 Tierno (just... not the best draw supporter out there)
-1 Great Ball (just... not the best pokemon search card out there)
-1 Escape Rope (again, from experience, only one is required - Lysandre & Switch are better cards for the effect)
-1 Max Revive (Aggron/M Aggron is a beast and will not die easily - also Lysandre's Trump Card is a better way of Max Revive-ing)
-2 Metal Energy (contrary to instincts, less is more with energy, and thanks to Bronzong's spectacluar ability you can function on less energy than you might think)
-1 Shield Energy (it's a fun card, but it's useless once it gets Enhanced Hammer-ed since Bronzong can't touch it in the discard)
-1 Kangaskhan (with all of the additions taken into account, you won't need a pokemon to use your attack for the turn to set up)
+2 Professor Sycamore (all about that consistency - a 4 count maximises your chances of hitting one and mitigates the effect of one being prized)
+2-3 Shauna/Professor Birch's Observations (or N, if you have it) (when Sycamore is too aggressive, shuffle-draw is your best friend)
+2 Ultra Ball (again, consistency - 3 Ultra + 2 Repeat is perfectly fine for searching in this deck)
+1 Repeat Ball (again, consistency - 3 Ultra + 2 Repeat is perfectly fine for searching in this deck)
+1 Switch (Switch is fantastic since so many Steel pokemon have insane retreat costs)
+1 Steel Shelter/Battle Compressor (consistency to win stadium wars/easy way to get energy in the discard - pick your poison)
+1 VS Seeker (best card in the format - HUGE consistency booster! run 3 if you can)
+1 Lysandre (amazing card to help secure a KO when you need it most - add 2 if you can find room!)
+1 Lysandre's Trump Card (not essential, but nonetheless a fantastic card for getting back resources - bit of a double-edged sword given that you also give your opponent their resources back, so use with caution)
+1 Roller Skates/Battle Compressor/VS Seeker (this one's up to you: Skates boosts consistency, Compressor gives another chance to discard energy/undesirable cards in your matchup, VS boosts you chance of reusing a supporter - all consistency boosters, but which one you use is up to you)
+1 Excadrill (Omega Barrage) (a 2-2 line is preferred just incase of prizes preventing you from having your Excadrill out in a match)
Any questions, feel free to ask! Looks like a good deck though
