I don’t know the elements but most DIY store elements are made of thin wood and nailed with short brackets; that’s why they often come loose quickly. I would completely skip paint, otherwise you constantly have to touch up and if it’s not opaque paint, you quickly see where the wood has darkened. I always have a hard time with such long and tall fence structures. First, they have to be set accurately and second, it’s visually a wall, even if it’s made of wood; I don’t like these long walls at all, even if they seem to be popular at the moment. You should probably separate the issues of sound and sight, because with such a measure there is no sound protection. At our former house, we also had a 20m boundary and we set wooden posts and then filled the gaps with boards. That is not difficult work and 20mm wood does not rot as quickly as the ones in the fence elements. For that you can take the cheapest wood you can get; I would skip paint etc., then it just ages a bit sooner... We had a slope protection made of pressure-treated wood for which I was told it lasts 4-5 years. After 12 years it is still there and lasts for more years. But in front of this board wall we planted thuja because you could get them quite cheaply in 100-140cm height. They are known as cemetery trees, but I liked them because it always looked like a big green cushion, eventually over 250cm tall :D and very dense; I thought it was great and really cheap as well. The board fence behind is still standing today (25 years) and was planted green on both sides. If I understand you correctly you would also like to work on it yourself, so I would look for ideas that I can implement alone; finished DIY store systems do not count for me.