I recently downloaded the "Midwest Central TS19/22 V2.1 June 2023". I then decided I wanted to make my own session for the layout, but when I go to save my current progress it wants to create a new route and save the new session. How do I save ONLY the session?
You can't, you have to save both then open the session config and change the map kuid and the one in the kuid table to that of the original route. You can then delete your copy of the route. It is your session so it also wants your route. Keep both until you have finished making the session.
Session creation works as it has always worked. Pick the route, choose view sessions, create session. Only make changes to the session layers and save the session without flipping over to Driver from Surveyor. Close Surveyor and open your session in Driver. What is tripping you up is you are attempting to test your session from Surveyor by switching to Driver. Once you do that and then flip back to Surveyor, you will almost always be required to save the route as well because something as simple as changing the direction of a turnout while driving changes the state of the route. When you flip back to Surveyor, the current state of the route is transferred from Driver to Surveyor where Surveyor considers the route to have changed. Whether this is a bug or a feature of the UDS interface is yet to be determined.