How are you testing the lumen output? What instrument do you use?
*I’m not sure that a 200+ lumen drop in ~30 sec. is just from high Tj (junction temperture). But with the rise in LED Tj your going to have a considerable decrease in lumen output over time. Nearly all heat produced is conducted through the back side of the chip (specifically the P-N junction). So every method of removing heat from LEDs should be considered.
Realize that you don’t have the best of cooling methods i.e. direct soldered LED to copper base, but what you have is an over driven LED mounted on an aluminum (AL) MCPCB, which is probably inefficiently attached to the AL heatsink.
For every 10°C rise in Tj, the luminous efficacy decreases by about 5% at constant current. Article. That’s why Tj directly alters performance and why proper heatsinking is of utmost importance.
So in conclusion… you’re like in “Turbo” mode dude! Therefore use sparingly.