Dear All seems to be a plural, whereas "querido" is a singular. I don't think the literal translation "Queridos todos" is a good option. Maybe "Queridos"?

Dear all = estimados (more formal or business-like). Maybe also "Colegas estimados" And Queridos for less formal / non business related. I would not use "Dear" in spoken language - it's what a much older person might say to a young person / child.