Good analysis, Diebroke...
How long ago were each of these cruises, especially Regent?
One advantage to a cruise assigned hotel is that you get access to the cruise line to moment you reach the hotel through representatives. Want to change a dining reservation, hire a car in a port, get a direct hotel to ship transfer, and have them tend to a pice of luggage you don't until you are onboard (formalish-wear) then they are there to do it.
But I agree you pay a premium for that, and sometimes you don't need any of those things.
I like your analysis of shore excursions. I like having a mix of pre-arranged excuriosn plus the liberty to go on my own.
With Viking River - the pre-arranged tours in Moscow included things I would not have thought of (The Moscow Circus, and military museum) and they were amazing. But I also loved the stuff we arranged on our own - the Yusopov Palace and the Museum of Political History in St Petersburg.
It seems a critical question here is...
How good are the included Regent tours?