Art and/or Architecture focused tour- multi day or one day.

Rocky Ruggiero is an academic who runs art tours in Italy, primarily Florence. (I have not taken one). His website is rockyruggiero.com.

Colleges and big museums construct these, for example the Metropolitan in NYC. They tend to be expensive, but get access to a few rare opportunities. I went on a Smith College Museum of Art tour to three contemporary art festivals, Documenta, Skulptur Projekte Munster, and the Venice Biennale. They rarely all fall in the same year.

I believe Context Travel offers single-day tours led by specialists (art historians, etc.). I haven't taken one of their tours, but the company has a good reputation as far as I know. It offers tours in several major European cities.

https://www.contexttravel.com/cities/london

I am a traveler that has visited 83 foreign countries and love art, architecture and history.
To me, Italy is the best country for these things, it is wonderful.

I am not an art expert, but have studied it and been to most of the World's best art museums. I don't think you need to take an expensive art tour.

Sure, do your research and if you find such a tour that is not outrageously overpriced, go for it. However, with a modest amount of personal study, you should be able to enjoy touring to focus on art, architecture and history.

I know many museums have great tours or have the option of touring with great audio guides. We recently did the Prado (again) in Madrid and there was a really good audio guide for that museum. Also, the Rijksmuseum and Van Gogh Museums in Amsterdam were set up to enjoy all the art in a knowledgeable way.

Many years ago (about 25 yrs) I took an art & architecture tour of Tuscany with the Minneapolis Institute of Arts. It was my favorite tour I ever took, We based in one town in Tuscany and did day trips to many cities and towns to explore the art and architecture, mostly concentrated on the Rennaisance period. The tour was 8 nights/7 days long and it was fun and educational. We had two tour leaders (docents of the museum) and we also had local guides in many of the places we visited.

So, I would suggest you check with your local art museums or schools in your area to see if they organize this type of tour. I know the Smithsonian offers some guided tours but I don't know any specifics.

EDIT TO ADD: I did see that Insight Tours offers an Art Tour of Europe - it's a very active, quickly moving tour that hits a lot of highlights from Italy (Rome, Florence, Milan) to Netherlands. https://www.insightvacations.com/en-us/custom-private-travel/art-history-tours

Sorry, no guided tour ideas but there is a book that is kind of fun for those wanting some thoughts when doing DIY travel.

1001 Paintings You Must See Before You Die by Stephen Farthing.

I've enjoyed having a look to see what the author considers a "must-see" before heading off to various countries. I've even become aware of some gems that I might not otherwise know about.

I googled "Art Tours of Europe" and saw the Insight Tour. It did not include the Orsay in Paris and only spent 1 1/2 days in Florence, so not a very complete tour in my opinion. If you love art, you need more time in Florence.

I really enjoy Ancient Rome Live, hosted by Darius Arya, Professor at the The American Institute for Roman Culture in Rome. He hosts multi-day tours to ancient Roman cites. https://ancientromelive.org/courses/. (I have not personally been on one of these tours.)

His Youtube videos are really educational, this turned out to be our favorite museum, nearly empty of visitors, on our last visit to Rome. https://www.youtube.com/watch?v=s6g75E6Lzls&t=1s. And you can check out his live lectures here - ($50 fee but will give you a good idea if you like his style.) https://ancientromelive.org/live-streams-seminars/

https://www.smithsonianjourneys.org/

Thanks for all these wonderful ideas! We have lots of info now to check websites etc.