Actually Summer BB isn’t a bad candidate for Buster Crit. She has her 1st skill for the Crit Up and Star Absorb, and her 2nd skill has insane buffs and her 3rd skill come with passive star gen on top of the card fixing.
I think her main issue will be her low-ish attack stat and lack of class advantage damage since Mooncancer is only advantageous against Avengers and Berserkers. Maybe you can use her with Merlin+Hans to ensure constant attack buffs and star flow?
A good AOE buster crit candidate who’ll be having a rate-up banner this year is Gilgamesh during Gilfest. He already has high star weight, star absorb and NP charge gain on top of Enuma Elish being effective against a lot of servant enemies so he can be used outside of his Archer niche.
A good ST buster crit candidate who’ll be having a rate-up banner this year is Quetzalcoatl during Christmas 2020. She has a similar skill set to Gilgamesh on top of insane crit damage due to her native crit damage up which can be stacked with Merlin.
Hope this helps!