That stocking list looks pretty good. I would suggest either albino/bronze or panda corydoras as they are easy to find, relatively undemanding, and look super cool.
I would suggest leaving the females in the aquarium and just letting them give birth there. This will be the easiest for you and the adult fish kind of "self regulate" how many fish are in the aquarium by eating some of the fry.
That aquarium looks amazing. I love the rocks in the front! Green tiger barbs are one of my favorite fish as they are a slight variation of the bulletproof tiger barb but have some different and super cool coloration.
Sometimes the fish have a preexisting disease and don’t survive qt with the meds. I know Cory has done extensive testing with this trio and all sorts of different fish including corys so I think the meds aren’t to blame completely here although the certainty could have been a contributing factor.
I have bred Bristlenose a few times and I always let the eggs hatch in the cave then transferred the fry to a grow out aquarium and put the male back in the display aquarium.
I had a cheap heater in a 20L with a pair of kribensis and it cracked/exploded and I was afraid it had killed the kribs but they were fine and I ended up getting a higher quality heater for the aquarium. Not a great story but just thought i'd share my experience with broken heaters.
I would follow the direction on the package as to how frequently you should dose. I think with easy green you shoud try to keep nitrates at 40-60 when dosing. Someone please correct me if I am off on those numbers.
I also dont know a lot about brackish however I know that mangroves can live in brackish and would also provide a cool root structure to add some interest to the aquarium for him.