Re Holly & Devin.
The show has never been shy about adding new characters, and technically Holly isn't new, but her being an actual character had been great this season. It's given Mike a chance to be a big brother. It's given the whole Wheeler family a chance to shine. Her story adds a fun mystery that syncs up so well. It's sort of like with Erica, starts as the (annoying) little sister, jumps right into the group proper.
And Devin. Little shit turned hero. Tell all the Whatsit kids to remain calm and then giving them a whack when it's their turn to leave. Seeing a Demogorgon really changes someone.
After the craziness of season 4's ending I was thinking, "Why would anyone stay in this town that's split halfway open?"
Patch it up and force them to quarantine! And then the main gang can run secret missions. It was perfect.