It can get very thin if you want to machine the diode and add in a regular lens. AtomStack is not really that great to be honest. Ortur for instance has changeable lenses which help a lot. So do other modules.
Some people are really into AtomStack, I’m not a fan at all. The Ortur brand seems to be well received, they have support in the forum even if unofficial. There’s no shortage of modifications including lenses, air assist etc.
Your AtomStack with air assist should be able to cut 3 mm poplar with no problems.
For ME personally, the build quality out of the box wasn’t good and I should have returned it on the spot but didn’t. They are supposed to have concentric screws to tighten the rollers on the rails and mine didn’t come with that. I can’t remove the nozzle so I cannot swap lenses etc.
If you are looking for an out of the box diode laser I would go with the most powerful Ortur you can get. There are others out there, but for a beginner I would say Ortur. They are not perfect but you can get a lot of accessories for them.
That’s a good question. Google is your friend here but the answers are complicated at best. You’ll have to go with the manufacturers specifications. You can test by doing a quick fire that you would normally use to align and then you’d need some very scientific tools. I would just practice with your laser and see if it works for you.