I'm so glad you discovered this before arriving in Europe, perhaps with long-term Florentine lodgings locked in. I was oblivious to the finer details of the rules myself at the beginning of my 2015 trip, and it was only thanks to a fellow language student that I learned how strict the limits were and was able to make the necessary adjustment to my itinerary.
Since you'll probably be spending some time in London next year, I'll mention something I discovered on my most recent trip--something I haven't seen mentioned in any guidebook. Royal St. is one of the streets cutting under the railroad tracks running south from Waterloo Station. By dumb luck I walked along that street to get to the Imperial War Museum from my hotel near the London Eye. As you cross under the tracks you encounter about two dozen ceramic artworks based on works of William Blake, who at one time lived in the area. They were created by a variety of artists.
Another thing that might interest you is the exhibition of winners from the BP 2020 Portrait Award competition. I don't think the dates have been announced yet, and it might open too late for you to see it, but I hope it won't. I've been to the last two exhibitions, both at the National Portrait Gallery, and I thought they were excellent. About 40-45 portraits, judged best in a blind evaluation. The 2019 contest had over 40,000 entries from 100 countries; no wonder the standard is so high.