I sprung for the Oase PondoVac 5, and have zero regrets. Yes, it was very expensive, but like you I have a tremendous amount of trees surrounding my pond. So. Many. Trees. My yard is basically a small forest. My pond is approximately 10k gallons, and does not have a skimmer (I am the skimmer), and has a bog waterfall for filtration. I enjoy being outdoors and scooping debris from the pond, however in spring when trees are dropping lots of messy bits, it isn’t possible for me to keep up. I do what I can, but plenty ends up on the bottom of the pond. First I rented the PondoVac 4, from my local pond supply shop, and found that while it worked well, it was a bit cumbersome. The PondoVac 5, is continuously vacuuming, and has larger wheels, helping me navigate my hilly and bumpy terrain.
I will have to typically vacuum once or twice per week for a few weeks in spring to get the bottom sufficiently cleared - which truly is great because it also serves as a water change.
I don’t know if you have a bog or not, but the vacuum has been especially helpful for me with cleaning my bog here and there in late summer and early fall. There is an attachment that minimizes the gravel that it sucks up, however all gravel that does get sucked up is caught in a bag. I then gently rinse off the gravel with the pond water being vacuumed and put it back into the bog.
It was a huge financial investment for me, but really helps me. My pond water is never crystal clear like others because of all my trees and the tannins that come along with that. I consider my pond water color more of a tea!