I like a coiled waist leash for the board because it is out of the way and easy to turn around in the water before getting going again. Leg leashes can bang on the water and get hooked around the foil or hooked around your feet.
I tried a 10' leash to avoid the dreaded upside down foil next to the wing thing. I've had one brand new wing commit harakiri on the foil. The long leash made no difference. The board rarely ends up at the end of the leash and can be upwind of you after a fall. I settled on the 6' leash which is fine.
I have a coiled wrist leash to the wing. I used to have a straight dyneema leash. The dyneema would get furry and hook up on the velcro on the wrist cuff and wrap itself around my hand. The slack in the straight leash would drape over my neck mid-gybe. The coiled urethane leash has none of those problems.
People complain about the coils hooking on themselves or between the two leashes. I have not found that to be a problem. The leashes might look tangled but any tension pulls them apart easily.
I have had one go with the wing leash attached to the belt. It works fine just riding. It stops me easily turning the belt in the water. I need to practice that a bit more before I come to a conclusion. Certainly wrist and waist is not a problem at all.
I have this leash
www.oneoceansportsaustralia.com/accessories/no-logo-waist-leash It's the cheapest leash and works just fine. I replaced the carabiner with the velcro end off an old Balin leash because it put a tiny scratch on my board. It's easy to swap leash parts around with a fin key.
The belt has velcro holding it closed. It's fine for just about everything. Quick and easy put on or take off. Occasionally I have worried about the possibility of the velcro letting go. Passing the velcro back through a protected loop would make it more secure and give an extra level of redundancy. The loop could be plastic covered in neoprene or a soft loop.
I'm not sure about plastic buckles. They're pretty reliable but if they fail they fail instantly. There's no partial fail state for them. They're also a risk of putting black marks on the board. I hate that.