First attempt at macarons. I got a bit of feet action, but every shell is cracked and flat. Tips for next time?

First time macaron maker here! Recipe from Preppy Kitchen

I sifted the combined almond flour and powdered sugar four times then set aside. Added the cream of tartar to my egg whites when foamy, streamed in the granulated sugar, and beat to soft peaks. Added vanilla and went to stiff peaks. I felt pretty good so far.

This is where things went south. After folding dry into wet (1/3 to start, then the rest), my batter was SUPER thick. Like, way way too thick. Practically a dough. I macaronage'd for a long time but it was barely changing texture, so in a last ditch effort I figured "if it's too thick, add liquid", beat another egg white until foamy, and added a portion of it. Once incorporated I had the lava-like figure eight test.

I piped the shells, waited an hour until they were firm and a skin had formed, and baked them at 300F for 14 minutes. Clearly each one is cracked and flat.

I'm guessing my problem(s) are in the macaronage stage. Obviously adding extra egg white isn't correct. Did I beat my meringue too far? Should I have run my dry ingredients through a food processor to make them even finer? Did I beat too much air out trying to save my thick batter and couldn't recover? I did not weigh my egg whites initially, so maybe I needed what amounted to 3.5 egg whites to begin with?

Overall I'm pleased with my first macaron attempt but would love any tips or notes for my eventual next time.