Lots of good input drom all sides thanks guys!
Regarding the kapton/thermal paste method vs COB holder, both methods have their ups and downs. If you prepare your heatsink properly (1000grit) and use a high quality thermal paste (thermal adhesive not necessary or recommended) you get the best thermal transfer possible. I put the paste on the back of the COB and place it on the heatsink, then press hard but carefully on the four corners and rub around slightly. Slowly the paste makes its way to the outer edges and as you keep pressure on it the paste gets thinner and thinner and creates a powerful vacuum effect. The COB holder would not allow for that, how do you know the paste has made an even, thin, bubble free layer? The kapton is only there to keep the COB from sliding sideways, not to keep pressure on the COB. This method also works great on 20mm stars.
Also, COB holders add cost/time to the build and if you decide to switch or combine 3070s 3590s and Vero29s, you have a mess of holes on your heatsink. I am constantly changing things around and it has served me very well not to drill my heatsink surfaces over the years. I have changed the position of my COBs already on several heatsinks when switching from active to passive cooling and I might switch back again.
The COB holder look awesome and avoids soldering which is very cool and for those who prefer them I say more power to you