I agree with @ATH - 20-30 feet is ambitious for my backpack sprayers. You'd have to position right underneath it and lob it onto the top, while avoiding the falling spray. Not my idea of a good time. Stilts will get you 3 more feet, which is not enough, plus you'll be wearing a backpack while on stilts...20-30' is a bit much for a backpack sprayer. Besides PSI which you can get enough out of the engine-driven backpack sprayers (doesn't look like you can from the battery-powered sprayers though), you still need GPM to build a column of liquid to reach some height. Also a tree that tall usually has some spread to go with it so you need several gallons to get good coverage. While 4 gallons will cover a lot of those small trees/big shrubs, it takes a while to get 4 gallons out of a backpack. Then you'll need to refill after every tree.
It can work, but honestly not the best tool for that job. But if it fits what you are trying to do, then I think Solo is the best option I have seen...but I haven't used it so no first-hand experience. A hand pump sprayer can easily get 15-20' on a calm day - again, you are going to struggle to get enough volume.