Places usually want the city tax in cash because otherwise they lose the percentage that the cc companies charge since they simply pass that money onto the city. I believe I have stayed at larger hotels that may just include it on the bill but smaller places will ask for cash.
Other places that generally only take cash are open air markets for cheese or fruit and I think it is generally better to use cash for smaller amount transactions like gelato and coffee. Taxi drivers really prefer cash over credit but are required to accept either.
Unless you run into unexpected larger expenses I think 150e is probably enough. Many people report going a whole trip using nothing but cards or their phone but I think there are enough <5e transactions that are still better with cash.
Take an ATM card and you know that you will always be able to get more cash if you want/need it and then don't worry about it.
Have a great trip,
=Tod