Not accurate statement that boric acid is not water soluble. It is soluble up to 5% (5 grams per liter). Although, this is not a HIGH solubility.Boric acid is not water soluble. Making your own is not so easy. If you use more than ~1/4 tsp boric acid per 8 oz water, it is too strong. Terro is a fairly good bait (boric acid). Amdro is better, in my opinion. Ants change their food preference based on their colony needs. That means sometimes a bait will work well, other times it may not.
Your pics are not good enough to identify the ant. A 30x magnification may be needed to properly ID the ant. I suggest you capture a few and take them to your department of agriculture, or whoever manages pest control in your state. They are free and they know your area.
Your location has relevance to what the ant could be.