Just 20nm away? $5?
What 569 or 609nm DPSS are you getting for $5 exactly?
Also 20nm is a lot of difference in that part of the spectrum. That is literally green or orange. You skip yellow entirely.
20nm is even enough difference to tell the least responsive part of the green spectrum apart. Why fly to New York when its only got an ocean between us? I'll swim as its the cheaper option.
Also wolfram, if it was that easy. I think many would of done it by now. Including those working in labs. Crystals need appropriate phase matching. They need coatings. And directly doubling diodes doesn't give as nice beam specs as pumping a solid state before it.