In general, in terms of settings:
Slower = darker
Higher lpi = darker
Higher power = darker
So the darkest possible is slow, hot and high lpi, but there are limits here; go slow and hot enough and you’ll burn straight through.
Also with plywoods the deeper you go means you hit new layers which can change the look of your piece in unexpected ways. Each of these factors that improve contrast also lead to deeper engraves, which may not be what you’re hoping for.
Beyond settings, understanding raster image prep principles and how to use curves to maximize dynamic range of your image can really improve the final result. If you’re not familiar you might want to check out #9:
And lastly, materials matter a great deal. For example, Baltic birch doesn’t engrave nearly as dark as cork, mahogany, maple, basswood etc. It might be worth it to find a more suitable wood species.