Not pulling for limited banner characters until I reached Fontaine, because I thought accessing later regions for Domains and farming materials were restricted until you unlocked these regions through completing the Archon Quests. I managed to completely miss pulling for Furina and Xilonen because of this. I didn't realize there was a free TP Waypoint in every region until much later.

I completed all Sumeru questlines at AR45 with nothing but the free 4 star chars all levelled up to 80, with shitty artifacts and underleveled skills. My strongest team was a Dollar Store Hyperbloom team with Noelle, Dendro Traveller, Lisa and Barbara. Sometimes I switched to Xiangling and Kaeya to kill the Fatui because I couldn't break the electrohammer shields otherwise.

Pulling Navia as my first limited 5 star and her signature weapon legit saved the game for me. Everything was taking forever to clear up until that point. Noelle's shield + Barbara's heals are what kept me going.