How long should I wait before starting the boric acid capsules again if I had a urinary tract infection, and is it better to use vaginal probiotic capsules before?

There is no specific waiting period needed between finishing a course of Sulfatrim DS and starting capsules of boric acid. If you want to restart the capsules, it is not a problem. As for vaginal probiotic capsules, while they are optional, some products have shown benefits in reducing the risk of bacterial vaginosis. It may be worth considering, but it is not mandatory as their effects are not comparable to boric acid.