What concerns public transport, yes, the STP Flex and the Tageskarte Gemeinde offer the same range of validity per travel day; not so, however, what concerns free admission to museums. The latter is not granting anything in that regard.
The Tageskarte Gemeinde is actually destined to residents and only available to residents living in the respective village (and paying their taxes there) and travelling spontaneously. Some villages do not offer the Tageskarte. The price is not set in stone and can vary from village to village. You would have to know a resident who could organise the Tageskarte for you from the municipalities’ administration office. And, depending on the village, the number of tickets available per day is limited; consequently, tickets are often sold out in advance. It is definitely not a ticket which you can use for multiple travel days. I suggest you look at the Saver Day Pass, or the Swiss Half Fare Card and the Saver Day Pass.