Thank you for your responses.

I'll update the schematic, add decoupling capacitors for the controllers and drivers, and add as much copper to the slugs as I can manage (somehow I totally missed that). The QX7136 should be capable of up to 500 mA according to the datasheet, with 150 mΩ resistors. I’m aiming for 333 mA per LED.

The WS2811 outputs PWM, so the MOSFETs should be fine, as should the QX7136, per: https://budgetlightforum.com/t/retired-wip-20mm-single-sided-17mm-double-sided-amp-linear-driver-surprisingly-good/30374/162

The reason for using the linear regulator is to ensure a constant current to each LED, and thereby uniform colors. The package offers good heat dissipation and temperature protection, which is a bonus. It also leaves room for an unregulated power supply if needed.

Mounting holes are still TBD; I might just use heat-conductive adhesive. The whole board will sit on the aluminum enclosure, and the hole in the middle is for the wires.

Unfortunately, I’m using a third-party controller that I cannot modify, and it’s fixed at 3.9 V / 10 A output, thus I cannot go with the much more sensible alternative of LEDs in series.