I’m using Cricut Infusible Ink markers. They were pretty much made for this, using heat to dye polyester-based materials, like PBT. I just have a 5w diode laser, and I do it “low and slow,” 2% power and 45mm/minute. Black and blue still work best, but the red (pink) and green can be nice too.

I have done two entire keyboards worth. The gray DSA i did in black are holding up great, but the legends went on wonky because I hadn’t refined my workflow. The next batch I did was on this same type of white XDA but while alignment was vastly improved, the ink didn’t go on as well. The only thing I did differently with these two was make sure to clean the caps with IPA first.