Worth it to add dregs?
If you don't like the resulting beer after you add dregs, then nope.
Acquiring additional Brett strains are the main reason harvesting dregs is popular, but it's by no means necessary.
One or more particular Brett strain(s) or a mixture thereof is responsible for the high attenuation of your blend. Wine yeast is used for bottling specifically because it won't cause further attenuation besides the priming sugar.
Suggestions:
-Stop using dregs? If you like the results, then great!
-Gueuze tends to be dry (more than straight Lambic or most/all other unblended styles), so maybe blending isn't the best choice if that's not to your taste.
-You could consider bottle pasteurization or sterile filtering to prevent further attenuation after blending.
-Consider a "turbid mash", crystal malts, or adding flour to the boil to try to increase FG by adding lots of starch.
-Higher ABV (8%+) will reduce attenuation.
-Non-fermentable sweeteners.
-Serve with a shot of syrup as is customary with Berliner Weisse. Or a sugar cube in the glass.
-Increase chloride and sodium (can also be done with finished beer) to reduce the perception of dryness.
-There are lots of possible sources of Brett or other wild yeast, everywhere. The vast majority are probably less attenuating than most dregs. Feeling lucky?
Cheers