OK, so it seems like there was some sort of issue that caused the finicky corals to die.
What I would do now is the following:
Do not feed the tank, just feed the fish (remember, corals are mostly photosynthetic)
Do not add any scleractinian corals (nothing hard), just zoas, toadstools, leathers, etc - this means you do not need to dose anything at all for a while
Do regular 10% water changes every 3-4 weeks - if the tank starts to develop algae you can do these more frequently
Test No2 No3, pH, SG, temp every week
I would follow the above and just be very patient. In a couple months if the soft corals are thriving and growing then you can add an easy hard coral, like some candy canes, basic (cheap) frogspawn, or a scolymia, NO SPS or torches until you are able to keep the LPS alive. At that time you will need to test a bit more frequently to see what the Ca/Alk is doing.