Search for software PWM.
I’ve done 72 channels on an 8MHz PIC in the past.
The only extra thing you need to add is the interrupt to reset the counter - just like it is in the existing code.
On a 16MHz cpu, you can possibly run the PWM cycles as fast as the loop() allows without any visible degradation. At least 4x the mains frequency is probably enough. Needs to be tested - esp with LEDs on the load. Incandescent loads will provide some visual smoothing.