Bob, I agree with you on Fastex requiring more finger dexterity and the way that could be an issue. On adjusting it though? Nope. I have a big heavy backpack with a gen-you-whine Fastex waist buckle, and I can hump that up on my shoulders and pull the waist buckle tighter in one easy shot. There are also "fastex-style" buckles that are meant for things like duty belts, which are not adjustable--those are meant to be set and fixed, not at all adjustable while being worn. From three feet away, they all look like. They're not.
For reasons lost in history, my current weight belt is actually about 14" longer than it need to be. And it got drilled into my head a long time ago that you never cut a rope (or belt) unless you have to, because it is much harder to lengthen it again. So, I keep the extra long "tail" tucked into the side of my BC. In the odd event that something flipped by buckle, the belt would still have to drag an extra second or two while all that tail pulled through, and that works just fine for me.
I was also taught to use a "nylon" buckle for my weight belt, and face it opposite to the direction that my BC buckle is in. Plastic because you can cut a notch in the leading edge of it. So, by muscle memory, there's only one buckle with a big notch in it, there's only one buckle that releases "from this side". And there's a long tail making sure that when I take off the belt (like on a boat) I can get a good grip on a good piece of it, before it is free of my hips. Intentionally making it easy--and hard--to ditch the weight belt.