I created a new simulation project called 'Test1':
More of my public projects can be found here.
Nice geometry! There are two quick things you can do in order to mesh it easily:
Merge all solids in to one. You can have a look at [this] (Merging solids in Onshape and meshing) forum post for more info.
You can than follow [this] (https://simscale-2.wistia.com/medias/1rceuszi3e) video tutorial to create a mesh. First try to create mesh with 2- Coarse refinement level.
Hope this helps. If you have any question/s, feel free to ask.