I still think iron fertilization is the best bet, not for co2 removal, but for albedo modification. Keeping equatorial kelp or algae mats alive would prevent lots of energy from entering the system. Those mats have albedo close to snow in the infrared, where water is about as absorptive as it gets.

Ive seen stunning simple / passive water column chimney-effect approaches to do this using lower water levels mixing, no sources though, was german tv (arte station)