Service at St Thomaskirch Church

The Sunday morning service at 9.30 am is free. You don't need a ticket.

Leipzig is definitely worth a detour! The Thomas Church in itself is special but there is quite a bit to see close by.

If you are interested in classical music, Sep 18 has a full schedule of events at the Gewandhaus Music Hall in Leipzig. They have 2 guided tours, opera, chamber music and sing-along event and more. It's part of a Mendelssohn Festival event that weekend.
https://www.gewandhausorchester.de/en/schedule/

We were in Leipzig two years ago. We weren't there on a Sunday morning but there was a Vespers service on the evening we arrived and it was lovely. Just being in Bach's church was very special. The Bach archive/museum is right across the square and is well worth visiting.

Shout out to Leipzig. Along with Hamburg, I would rate Leipzig as one of the most under-rated large cities in Germany (at least to North American tourists). The city center doesn't take long to explore, but it's worth at least 24 hours of your time.

In addition to the Thomas church be sure to visit the Nicolai church while in Leipzig. It has hundreds of years of history, but perhaps is most famous for its late 20th century history. This church was the home for and the origin of the Monday night candle light marches that led to the removal of the Wall between East and West Germany.

Thanks everyone for the advice. I added Leipzig to my trip. Especially with the Mendelssohn weekend, I've deleted my time in Berlin as I feel that being at Sunday service listening to Bach's choir in the church where he is buried would be a much more exciting experience for me!!!