We are visiting in early August and plan to take the train from Garmisch-Partenkirchen to Munich. We plan to use the Bayern Pass. I can just buy it there before hoping on a train, right? Will there be availability on all/most trains in the middle of the day on this route? Or is it better to pay for a full ticket and make reservations (the full ticket is only a few euro more since it is just 2 of us, but I would like to have the flexibility of not locking into a time, and the chance to split the Bayern-Ticket with other travelers).
Trains between Garmisch and Munich are all local trains, so you can't make a reservation anyway. But I would expect they have plenty of availability.
With the walkup price of 16.90, the Bayern-ticket is your best deal as long as there are 2 or more of you riding together. You can get it at any ticket machine for 27 eur (or 29 eur from a ticket agent).