Yeah I know 1024 was pushing it, I just have a habit of working with big numbers and then scaling them down. I think I got the build time under control now after fiddling with my PC, although it still tends to get stuck in some cases for no particular reason.
I think I may have solved the awful shadowing issue as well, I don’t know if this is the most optimal way to do it, but it seems to be working: I simply went to the speedtree mesh assets in the content browser and got rid of the ST lightmap UVs and generated new ones with standard settings. After doing this I’m getting what seems to be pretty accurate shadows and none of those black artifacts. I tried this out on other trees as well and they seem to be responding well too. I’m about to make some grass for use with the foliage tool so I can only hope it doesn’t have as many problems as the trees did.