For the stuff on the bottom I think vacuuming is the best option.
Several vacuuming options. Removing the muck can help with clear water but is by no means always a solution.
Could be suspended particles. If you look really close at the water do you see lots of tiny bits, like ground black pepper size, kind of floating and suspended in the water? Vacuuming can eliminate that over time. For faster results water changes are needed. One 50% change isn't enough. Some types of vacuums remove water from the pond, so that counts as a water change too.
It can be hard to describe water. Murky can mean many things. You didn't say green so I suspect a second UV wouldn't help. I would focus on the current UV and make sure the bulb isn't too old and that the flow of water through the UV is within the manufacturer's spec. And the UV is mounted correctly, not upside down, etc... If the UV is sized correctly, maintained and installed correctly it is 100% effective. Adding more UV filters doesn't do any good if that one is also not done correctly.
I'm guessing the pond has no string algae? Kind of common in your type of pond. You could add a Trickle Tower filter in a spot getting as much sun as possible. Get some string algae growing on that and very likely you could have clear water. And you wouldn't need the UV at all.
It's not hard. But you have to be pragmatic and careful about advice. The things people will tell you to do can be endless. This includes local pond "professional". Good luck.
