I gave t another trywith just three schemas selected. That works - more or less.
Toad goes into wait, establishes a connection, goes blank, is reported by Winodws as not responding, but Toad perseveres, and, ultimately, comes back to life with all three sessions established.
I can imagine that opening multiple sessions may take some time, yet the behaviour is a bit awkward. How about a little pop-up showing progress?
Some of the intermediate stages should be avoidable, imho, such as going blank and/or not tresponsive to the OS.
Just some thoughts…