This is definitely speculation, and this is a stretch, since you didn't clean off the old thermal paste you'd think there'd be 2 sets of dots (thermal paste) slightly offset. Which suggests that the new LED CB didn't come with any and when you tightened the new one down it just transferred the little bit that remained on the heatsink.
Although, it's also possible that application of the dots (thermal paste) is done by a machine and you won't be able to know for sure unless you did clean the old thermal paste.
After you put it back together, and the fixture is cool, do the lights turn back on? Or does the fan just kick in full speed?
The reason why I ask is that this may be more than a heat transfer issue, and something more catastrophic. And if that's the case, the only help you can get is from ecotech directly, or if it's out of warranty, I know that
https://www.fixreef.com/ will get that light working.