I think all we can offer is opinions, and most opinions are valueless without real, proveable info to back them up. We're guessing.
Scenario one: the established, survivor fish carry a pathogen the other fish die of, but that they have immunity from. This is common when we add wild fish to tanks with disease survivor farmed fish. The farmed fish live on while the wilds die off quickly. It could happen the same way with farmed fish from different systems.
Scenario two: a poisoning that some species are affected by, but not others. That would be odd.
Scenario three: a slow working pathogen affecting the most stressed fish. Angels is those number in that tank will die young. There is no avoiding that. It's what overcrowding does.
Scenario four: a series of unfortunate coincidences. The Bettas dies of one thing. The gouramis died of another, as they are prone to do. The stressed angels are dying of a third thing.
Scenario five: (You'll have this info) the tank wasn't well maintained, and water changes stretched over a week, then to 10 days, then to 2 weeks and onward, and the stressed fish fell victim to previously controlled diseases they carried. We love those water tests and they can be useful, but they are extremely limited in what they really tell us.
You can choose an answer you like, but will it be correct? All but scenario 2 make sense to me. So what to do? I would water change heavily, for a few weeks, and water change normally, every week, forever. You've already realized the danger of impulse buying. So rebuild slowly and steadily, researching the needs of every fish before you buy. We say research and it sounds fancy, but a run over to Seriously Fish and 2 or 3 other sites and spend half an hour reading about the species that interest you.
Ask about member's experiences with the fish here after you've read up, and see if it all makes sense to you.