Based on what you have expressed. With the assumption that you have the time and means to travel several times over the next decade (or linger).
Go on a cruise out of Copenhagen that visits Sweden, Finland and Norway (probably goes to Kiel or Warnemunde). okay - that’s a Baltic cruise, isn’t it? It might not do all three of your choices, but look for at least two of those at first.
Plan for a day or two pre-cruise in Copenhagen. Spend an afternoon / evening in Tivoli Gardens (inspiration for
Disneyland). Hotels can be fairly reasonable near Nyhaven - a canal area with colored wooden buildings. It is walkable (a few blocks to the city center and to the current monarch‘s castle). Food is expensive. There’s a UNESCO world heritage site about 1.5 hours away. Malmo, Sweden, is across a bridge.
(I was in planning stage - we had a very nice, active group on Cruise Critic; then the itinerary was cancelled when the ship was re-assigned).
If you don’t want long transports inland for the big cities, just find things to do in the port city area (Warnemunde is a long way to Berlin. Kiel is used for Hamburg). But a stop in Oslo is pretty much right there.
If you are okay with just scratching the surface (and the perimeter / coast of a country, then doing an ocean cruise is one of the easiest ways to sample the world).
I did an 11-night Med in Disney in 2010. A lot of ports. We combined ship tours and DIY activities. It was tiring at times - a shorter cruise, but with more return visits to Europe, would help with that. (I did go back for London and some France. Revisited London/ Bath /Stonehenge /Paris on the way to a second TA cruise and Barcelona). It is the thought that I can return (even if I don’t) that makes a cursory trip sufficient to at least see a little of a country - I am not going to see it all, ever. Most of us don’t even see most of our own country.
Yes, lugging bags from hotel to hotel is not the most fun way to travel. Although I would love to do some train travel. That I would save for a trip to one region or area - perhaps a trip to Italy to visit Rome, Florence, Venice.
Dipping your toes back into European travel with a cruise should help you determine your comfort, fitness, and interest in future trips. I find travel spurs me on to want more travel. I am 62 and looking forward to more adventures.
I think just picking a general area for your First trip will help. Thinking it won’t have to cover all your travel wishes. The fact that you actually stated an area that interests you is a good location around which to travel. And you posted on a cruise site. Sounds like you have a good idea of what you want.