Can share which European cities you have been to?
Btw, visiting Heidelberg is not like visiting Rome. Likewise, visiting London is not like visiting Cambridge.
I'm not trying to change your way of travel. You can do whatever you like. Just sharing my experience with Rome. I was in Rome for 4 days and I wished I had more time. I did Florence and Pisa too.
My recent trip to Europe was during the Spring season
Overnight direct flight to Amsterdam. 2.5D Amsterdam (including Lisse). Day 1 City area, Day 2 – Visited wind mills and tulip fields. Next day took an early morning flight and reached London at 7.30am.
2D London city.
Train departed London before sunrise and arrived Paris at 8am.
4D Paris & Strasbourg. Day 1 – City area (Eiffel Tower, Louvre, Arc of Triumph, Notre Dame Cathedral & etc. they are all nearby) Day 2 – Montmartre, Day 3 Palace of Versailles. Cruised Seine river during sunset - Day 4 Strasburg. Next day early morning took a train from Strasbourg to Swiss.
4D Interlaken (including Jungfraujoch), Zermatt, Lucerne. Directly flight from Zurich to Sg at night.
Travelled light and locked bags at stations before Airbnb check in time. All accommodations were within walking distance from metro/train stations.
Started each day early in the morning and went back to accommodation at around 9pm sunset time.