Personally I think you just need to slow down. You started this tank on 1 march and in 7 weeks you have cycled it with all new rocks, added at least 4 fish, including a tang and a fair amount of corals. You have a serious issue with diatoms or yellow cyano
type bacteria. When your first corals died you suspected it was possibly due to water quality issues, which leads me to believe you have known water quality issues.
Starting a new system right from scratch as you have is how I will start my next tank, but it is the slowest way to start. 6 weeks of cycling before the first fish goes in. IMO
you need to slow down. No new additions except perhaps some more cuc. Get the bacteria issue in hand. Let your tank stabilize a few months then start spending more money in corals.