I'd add black neon tetras as the next school. And definitely up the group of cories to 8-10.
I found another two concolor (slate) corydoras and I have two more ordered - that will take my corys up to 10. I'm striking out finding harlequin rasboras which are my first choice for my next school of 10 or so. Are they worth waiting for? What about rummynose tetras? What do ya think? Harlequin Rasboras vs Rummynose Tetras?
(reminder: there are also 8 cherry barbs in this 29 gallon)
Can your LFS special order the rasboras? You'd be fine adding rummy noses instead, IMO. They're one of my favorite schooling fish.