Hi Scott, that's great to hear.
Your laser does not support TTL but it can use pulse width modulation to switch the MOSFET on and off quickly which creates a variable power. It's not as linear as a true TTL module but as you've found, it works.
I'm glad you've found velocity mode, it's actually a better solution to getting grey scale with a non-TTL laser as it's very linear and can even work better than true TTL. You want to set the rapid feed rate at a level where it's not quite burning and then the raster feed rate where the burn is dark, the wider the margin the better the results will be.
3500 is about right, especially for scanning as you'll start to get backlash when the belts stretch no matter how good your machine setup is.