I would print water-transfer decals, and apply to the front of the keycap. Because I would never touch that surface, I would not have to care about wear.
It could be fiddly to apply each decal, but it would be inexpensive. I would use additional space on the paper for printing symbols multiple times so that I would have spares of a symbol in case I would mess it up.
Unfortunately, that would not work well with all types of keycaps (such as EnjoyPBT) that don't have perfectly smooth sides.
That printing you saw in the video, I would call "laser-etching".
Most makerspaces don't permit ABS plastic in their laser-cutter/engraver. ABS melts easier than it etches and the fumes contain cyanide, which is extremely toxic. You would need industrial-grade ventilation. Both PBT and ABS keycaps are laser-etched and laser-engraved industrially. However, from observing people use the laser-cutter at my local makerspace, I know there could be a lot of trial and error before you would find useful settings.