So The parts library I use ( the one I linked) has little sketch points in the middle of all of the holes on the c-channels, that make it really easy to joint them together- you don’t have to worry about clicking on the edges or anything like that. With screws, gears, and all of those, you don’t need sketch points because you can just click on the circular holes.
There are also a lot of good libraries for autodesk inventor, which also has a student license version. Lots of people use that, and say that it is very good, too.
If you are looking for something that is focused on vex, you could always wait until the download version of this is ready: