Don't be too quick to think that there is something seriously wrong with the adjustment of your traces if they 'slacken' or droop just a bit when you are slowing or stopping; that is what 'should' happen, esp. with properly-adjusted breeching!(If they didn't 'droop' at all, it might mean your breeching was too tight...) Yes, it would be a bit more evident with bio/bio beta harness, which IS 'stiffer'. When adjusting your breeching as you hook, be sure that the vehicle is 'pushed back'(for want of a better term!),so that the traces are NOT 'drooping' and are as fully extended as you can get them without the horse actually being in draft(draught)-which just means that the horse is actually in the act of 'pulling' the vehicle and its load. For the best/most proper adjustment of the breeching, you want the horse standing squarely between the shafts-not have its body 'angled' (this is not as much of an issue with a Frontier EE, or most 'straight shafted' carts, even such as show carts and/or road carts, but more of one with a cart like my Bennington, where the shafts come 'straight forward' for a ways, then 'turn in' to be closer to the horse's side at the point where they lie in the tug loops-this configuration allows the horse more room to move their hips over and helps to accomplish tighter turns-by contrast, most 'straight' shafts taper inward toward each other continuously from the attachment to the cart up to where the shafts lie in the tug loops. I believe the Bellcrown,the new Smart Cart, and probably the new IteBte, also have this kind of shaft configuration-it truly DOES allow the horse to turn more easily--but also requires closer attention to the proper adjustment of the breeching, in my experience.
Another thing to keep in mind is the height of the tug loops. Remembering that you want the shafts to 'float' in the tug loops, you want to keep in mind that if they are too high, the shaft tips(of straight shafts, at least) may 'poke' the horse in the neck when turning; if they are too low, it will be even harder to achieve the proper balance of the cart(the 'float')when the driver is in the cart--and the shaft tip may 'poke' the horse in the shoulder...and the horse probably won't like EITHER of those scenarios. (You can actually 'see' the shafts 'floating'; if they are, even with wrap straps, you should be able to tell that the tug loop hanger straps are slightly 'floppy', whereas if not(as when you sit, or lean, forward), they will clearly be 'taut', meaning there is considerable WEIGHT actually being bourne in the tug loops, and therefore, on the horse's back via the harness saddle.
So, don't hesitate to keep on 'tweaking' as you critically observe how things go as you drive!
Happy driving!
Margo