I am working on a controller as well and noticed you were looking for a way to print the letters on the buttons. I am going to use this product to print out the labels and put them on my controller (Which will be a face plate of 1/8" aluminum I am going to mill out). After printing they can be sealed with a UV sealer for added protection. They are cold transfers: Print them, put them in lukewarm water, then slide them on. Basically just like model decals from the old days, just custom.
https://www.transferpaperexperts.com/translide-clear-laser-transfer-paper
Hope this helps.
Keep up the good work!
Ben