This sauna right at the lake Tegernsee is a true gem and some of us had already know this sauna from previous visits. It is well reachable from Munich Donnersbergerbruecke via BRB trains (included in 49 EUR ticket) and the proposed 4h slot from ~1730 on means we can reach one of the final trains back to Munich, leaving 21:52 from Tegernsee.
Note: you need a reservation for the weekends, book your guaranteed entrance reservation here (had this before, you will need to wait or cannot access at all - DO get a reservation!): https://www.monte-mare.de/tegernsee/4-stundenseesaunabuchen Book Arrival time 17-18:00 - 4h Ticket needs to be purchased on top at the entrance (31 EUR)
Train from Donnersberger Bruecke to Tegernsee (board the right part, most likely rear part) Donnersberger Bruecke 16:08-17:09 Tegernsee (last station)
Trains back to Donnersberger Bruecke 21:52, 22:52 (last train!)