Total water volume is 38 gallons. That's a lot of water volume although there will be a displacement caused by substrate, decorations and such anyway.
You could add more corydoras. Which species are you getting? The smaller ones such as Corydoras hastatus are quite a cute choice. Your tank surely can fit over 15 of these cute fish. Or maybe at least 8 Corydoras sterbai. Sterbais are very beautiful to look at and can tolerate high temperature very well. If you want the challenging temperate ones, Corydoras panda are a wise choice and your tank can fit at least 15 of them.
The assassin snails are bound to breed in time especially when you feed them adequate supply of meaty foods such as bloodworms, ramshorn snails and MTS. In my experience, the assassin snails took 3-4 months before the babies eventually showed up. The babies tend to hide a lot so I would not expect much in the first few months after you observe them laying eggs.
You could certainly add more cardinal tetras. If you want another variety, you can't go wrong with diamond tetras, Columbian tetras, lemon tetras or X-ray tetras. They are nice to have especially the sparkling colors of the diamond and Columbian tetras.
As for female bettas, there is a tendency one of them will dominate over the others so in cases like this, adding more plants will help tone down dominance orders and allow the weaker ones to seek refuge.
Hope this helps!
