I would investigate sticking as many 4ft. EB/samsung strips as you can afford on as many hlg-600h-24/48b's as you can afford.
The achilles heel of cob builds is that the heatsinks cost almost as much as (or more than) the cobs after shipping and dont necessarily get cheaper in bulk as the shipping is still brutal no matter how many you order.
If you were to order 500ct of the samsung 3000k h-series they are almost half the price as 1-9ct. The "heasinks" are as easy and cheap to source as going to your local metal supply and buying aluminum angle or channel.
Full disclosure: i have never flowered with strip lights, only vegged as i have more cob-based flowering rigs than i can currently use. But if i was building a flowering lamp tomorrow I would without a doubt be building with strips and not cobs.
Yes I have my own transforer and I turned in my load data sheet to the power company and after I pay $2500 to upgrade my transformer from a 25 to a 50 something or another I will be around 90% of capacity with the 48- 420W lights and other stuff like dehumidifiers, pumps and A/C. I asked if there was another level of transformer if I wanted more power and he said the lines would need upgraded and would cost more than I could afford. Does anyone have any thoughts on my COB density? Please chime in