Add constant current to an XL6019 module
You cannot drive 6 LEDs with 12V.

With 12V in and 6 LEDs you do not need boost buck, just boost. 

You should use a $7 Mean Well LDB-300L boost driver rather than the CV 12V source.  Its size is about 30 mm x 20mm 



You would need to use LEDs that have a max current of 300 mA or higher.  They also have 350, 500, 600 mA models.

With a 12V input it can drive a string of LEDs with a Vf of up to 36V. 

The LDB-300L has a PWM dimming pin that accepts frequencies between 100 and 1,000 hz. 

Mean Well LED Driver Catalog