Hi Patty! Just finished the GAS Tour last Friday and had a wonderful time. My guide was Carlos Meissner who was superb.
I recommend getting to Trier at least the night ahead if not 2. One couple missed the first full day (walking tour of Trier and the trip to Burg Eltz) as their flight was delayed then cancelled. I flew in to Paris for a few days then took the train to Trier via Saarbrucken.
I stayed 2 nights ahead and stayed in the tour hotel which is an Assisted Living facility with hotel space. Do plan to take a taxi from the station and have the address written down as well as a googlemap route printed out. Your taxi driver will likely not know the hotel.
I had a number of things on the list to do in Trier and got to some of them. Frankly it was SO hot in Trier (95!!) that I was finished with sightseeing by mid-afternoon. You won't have that problem with the time of year you've chosen. Do plan to see the Konstantin Basilica and the Dom at least on the Saturday as there is not time on the walking tour and it will likely be a Sunday so they will be having services.
I had done the 21 BOE in 2014 and loved Switzerland so was looking forward to 3 nights there. It was fantastic, as anticipated. We did the Schilthorn and hiked from Almendhubel which is just above Mürren on the first day then did Trümmelbach Falls as a group the next morning with most of us making our way up the Wengen side of the valley to do the Mannlichen to Kleine Scheidegg hike in the afternoon.
On your night in Hallstatt I recommend just taking a small bag for the night. The bus drives thru to the far end of town and drops off. Then you and your bags board a boat for a short cruise on the lake (maybe 45 minutes??) then you offload near the hotel. However, the next morning you have to walk back thru town with luggage to get to where the bus is parked. It would just have been easier to have a small bag. In Baden-Baden the bus can get pretty close to the hotel so no problem.
I love to talk about my trips so let me know if you have specific questions. I am still traveling but should have fair wifi. I'll do a trip report when I get back the first of Oct.