G2 is a different lens. It has a slightly larger spot but is about 30% more powerful (and more expensive). But definitely worth the cost in my experience for engraving and cutting.
620mm/m is most likely way too slow. I am sure the guide said to set it at 5v and then around 2amps. I run mine a 2.2 amps i think.
I run my settings at depending on the material at 0-255 power, and 3000-4000 mm/m. Many people I have spoken to also get decent results at half power and half of that speed.. around 1800mm/m.
Getting wood burning to look decent has been a long process for me and I am still working on it… but I am close.