I've done 10 RS tours but of your choices, only the 21 BOE. I did this one as my 3rd tour and combined it with Best of Paris and Village Italy.
I loved the 21 BOE! Before I took it, another RS guide told me he had gone on this tour as an assistant and loved it because there were blockbuster sights every. single. day. He was SO right! This is a fast moving, active tour. There were 2 days where we did 10-12 miles on just on the group sightseeing (Amsterdam and Rome). I loved that we got the taste of so many different areas - landscapes, architecture, culture, food, art, history.
Since you've done the GAS tour you'll have a little repetition as you'll stay in the Lauterbrunnen Valley for your Switzerland nights. The year I did the 21 day tour we stayed on the valley floor at Stechleburg but I think they might have moved those overnights to Muerren where you likely stayed on the GAS tour. I actually took the GAS tour BECAUSE I loved Switzerland so much on the 21 day tour so this is not a disadvantage to me! Neuschwanstein will also be a repeat. Since I'd been there on the 21 day tour I walked around the lake when we visited on the GAS tour. I enjoyed that very much.
One thing that was fun on the 21 day tour was that with the 3 weeks with one group we had a stronger bond than any of the 2 week tours I've done. I am still FB friends with a number of my tour mates 4 years later.
Since it was early on in my RS "career", I didn't do as much research on the sights in different locations as I should have but still managed to see quite a bit in my free time.
I went the end of August. I've done that time frame twice, for the 21 BOE and the GAS tour and I won't travel that time frame again. There is too much chance of hot weather which I don't tolerate at all.
I'd recommend adding several days in Paris at the end. The 1.5 days you have on the tour is just enough to get you oriented to the Metro system and there is SO much to see. I'd been to Paris in the 70's and had no desire to go back. Just didn't like it. I had decided before taking this tour there must be something to Paris I missed all those years ago so scheduled the Best of Paris to follow the 21 BOE, plus another week with a friend. I couldn't believe it when at the first night meet up the guide said - By the way, if you've been to Paris before and hated it, I will make you love it. Well, not sure if just his saying that led to me loving Paris or if he just taught me enough that I felt comfortable but I've been back nearly every year, sometimes twice!
You can't go wrong with this tour!