You have 6x 50-70V drivers right? So you buy 6x 50V COB or 6x Vero 29 C (68V) or 12x Citizen 1212 (maybe some other 36V COBs also would fit, but I only have charts for these right here).
50V COBs: Vero 29 B, Citizen 1818, 1825, Luminus CXM-22, CXM-32...
I would go with the cheapest option because of your drivers. At low currents like yours difference between COBs is not that much big.