Thought I'd post an update
Much has happened, since I steered more and more toward tannin stained water. In the end the only plant that worked to my satisfaction was the Brasilian Vesicularia Montagnei.
Current population:
4 L 091 (Rio Venturari)
4 L 075 (Amazon Basin)
6 L 201 (Rio Ventuari)
20 Corydoras Duplicareus (Upper rio negro)
200 Cardinal Tetras (Rio Negro/Orninoco, I added 100 and they didn't school. So I added another 100, and they started schooling)
I installed a Maxpect Gyre 250 to increase the flow/oxygenation. Its a monster, and I run it on only 10-20%.
In the end you could call it a "North West Amazon" bio. Or a mix of Orinocco Basin/Rio Negro. I'm happy with the result, and life is happy in the tank. I will start mixing in RO water this summer, and up my game a bit.
Here are the pics after 18 months after setup:
I'm working on a new 360L Rio Xingu setup, which I will post about shortly. Also did a Nano Borneo "bio" with 6 Brachygobius Xanthozonus. Bonus pics: