Welcome to the black art that is astrophotography!
Focusing with the viewfinder would be very difficult to say the least, the best way is to go into Live View and then zoom in on a star and get that as small and tight as possible.
You are best not imaging with a diagonal anyway. Going back to using live view and the back screen, some people get a DSLR that has a flip screen for this purpose.
You’ve made a good start
