Baking – make the crumble topping in a mixer

I agree with rumtscho that your dough is probably way too wet, but I'd like to make a few other points as well:

You don't necessarily need to add extra flour just because your dough is sticking to the bottom of your bowl. And definitely don't add any flour until your dough is fairly smooth (unless you already have a feel for the recipe). The flour will absorb a lot of water as you work the dough, so dough that seems way too wet after one minute of kneading may be fine after six.

Also, I've found that with the hook design in the Kitchenaid, dough frequently sticks to the bottom even with proper hydration. Just use a scraper to turn your dough out of your bowl and give it a fold to make sure that there isn't an underworked spot on the bottom.

As breadcrumbs in a savoury topping brown quite nicely (and so does unsweetened pastry) I should think you'd be able to find a mix that would work.

The recipe would probably involve both fat and some sort of starchy ingredient(s). You might need to switch from oven to grill for a few minutes when it's cooked to get enough browning -- and getting just the right amount of browning might be harder, as some ingredients might burn quite suddenly. Of course, only browned sugar tastes like browned sugar (unless you can find a suitable caramel additive).

Perhaps a good place to start would be a "flapjack crumble" recipe, modified in a similar way to the many sugar-free flapjack recipes out there. A crumble topping that's more like pastry crumbs should also do work but the texture is likely to be less forgiving.