In my past experiences in Europe I packed way too much and suffered for it.
We will have a car for 10 days then I will be on my own.
I was thinking of just taking 2 pair fast dry shorts and 4 stay dry t shirts. Sandles and one pair of socks. I will just do a lot of washing. Can I get into churches and museums with this set up?
I may take a wind/rain proof jacket as I will be a few days in Frankfurt. Maybe an umbrella. This will be for the month of Sept.
Frankfurt in September can be cool and rainy; you'd probably be happier if you added one pair of long pants, a lightweight sweater or fleece and a pair of leather shoes (not sandals) as well as the rain jacket you mention; if it doesn't have a hood, bring a hat.
In the past 3 years September actually was one of the nicest months in Frankfurt. Mild, sunny, no rain... temperatures in the upper 70s to mid 80s... But of course nobody can really forecast the weather for this year's September...Personally I wouldn't bother carrying an umbrella with me all the time. If there is rain when you get to Frankfurt simply buy a cheap umbrella or borrow it from your hotel. You can buy a cheap umbrella for less than €5.
Museums don't really care what you wear. But many churches do care. A lot of churches will not let you in with shorts or sleeveless shirts.