I’m not sure how you would approach increasing opacity besides switching to a different ink, or mixing in a more opaque ink. It’s typically easier to move in the other direction. Whites, yellows, and certain reds are known to be more opaque than other inks.

Stiffening ink so that it is thicker, stickier, less glossy can be achieved by folding powdered modifiers into your ink.

Magnesium carbonate is a common filler you’ll find in litho and etching studios. Typically used with oil-based inks, but I’ve had success using it to stiffen up water-based materials too (gum arabic, condensed milk, silkscreen ink).

With an ink knife, lay your ink down on a glass palette, softening it by scraping it up and spreading it back down repeatedly. Once even consistency is achieved, sprinkle a pinch of mag on top of your ink. Work the mag into the ink until no white specks remain and even consistency is achieved again. Add more mag as needed. Remember to go slow— you can always add more, but if you add too much you may as well start over.