Ok, glad to see you've got nitrates. Try not to overfeed, especially when cycling.
Yep, "it all started with one betta..." same here. Lol.
I am sorry for your loss too. But please don't give up. It happens. It's a learning experience. Been there, done that, learned a lot! You have a really nice tank.
As long as you have at least 360 gph total filtration, yup! Where is the photo/vid of the corys?