Updated "new" rough 4 month Europe itinerary with 2 teens

I still feel like Berlin is short and you have--comparatively--more time than you need in Munich/Salzburg etc. The latter depends on how many side trips you plan to make, obviously.

If you haven't done so, check the schedules for your intra-European flights. Those are fairly unusual routes, and you may find that non-stop flights do not operate every day of the week. Some are seasonal, and I don't know how long the "season" is. Those schedules may require a bit of fancy footwork, and it would be best to know the constraints ahead of time.

If you find you can't get a direct flight, be sure you know whether you're buying a true connection (so that you're protected if a delay on the first flight causes you to miss the second) or just two separate tickets (meaning that you assume all the risk). Connecting flights are likely to mean substantially higher costs as well as additional travel time.

My reaction is ambitious but not crazy. Sounds like you have the right mindset to slow down if needed. As long as you do not have an endless string of 2, 3, 4 night stays. That is exhausting. And it gets tiresome to eat in restaurants all the time. Having apartments will help. In fact, even though it is only 3 nights, I would get an apartment in Venice because I think you will be sick of shipboard dining and may want to eat less formally, have some extra space, etc.

If I changed anything else it would be to drop Poland and add time is Switzerland. Poland is an outlier and your time in SW is so short. Do you know where exactly you will go in Switzerland?

Even with a trip of 4 months, you still can’t do absolutely everything. In fact it’s desirable to leave some ‘must dos’ out, in order to inspire and motivate future trips. Remember you are still young!!

The length of your trip will also dictate the style of travel. You will of course look for more nights at each destination, avoiding one, two, even 3 night stays. You will want to move on ‘down the road’ rather than jumping about a lot. Hence you will most likely prefer a train to a plane. Lastly, look at compromises that will make things flow better, given what I’ve just described. So my advice would be to substitute the Amalfi Coast for Santorini, and Prague for Krakow.

Day
1..Madrid - 5 nights
6..Andalucia - 8 nights
14..(Fly to) Naples & Amalfi Coast
 - 5 nights
19..Florence & Tuscany
 - 7 nights
26..Rome
 - 6 nights
32..CRUISE - 12 nights
(Livorno, Genoa, Marseille, Barcelona, Malta, Corfu, Katakolon, Hirakleon, Athens.)
44..(Fly to) Switzerland - 9 nights
53..Munich & Bavaria - 6 nights
59..Salzburg & Vienna - 6 nights
65..Prague - 4 nights
69..Berlin - 5 nights
74..Rhine or Holland or Belgium - 4 nights
78..Normandy - 5 nights
83..Paris - 6 nights
89..London

After consulting more with the family, we've narrowed things down a bit more and here's what we've got. Two places that remain musts are Poland and Venice!

Day 1 Arrive Spain
Day 2-13 Spain (Madrid and Andalucia)
Day 14 Fly to Florence
Day 15-21 Florence/Tuscany
Day 22 Train to Rome
Day 23-28 Rome to include 1 day trip to Pompeii
Days 29-41 CRUISE: Livorno, Genoa, Marsailles, Barcelona, Malta, Corfu, Katakolon, Hirakleon, Athens
Day 41 debark cruise and train to Venice
Day 42-43 Venice
Day 44 Venice; evening flight to Krakow
Day 45-48 Krakow, Auschwitz, Warsaw
Day 49 Train to Berlin
Day 50-77 28 Full days for Berlin, Munich/Bavaria, Black Forest and Salzburg, Austria area?
Day 78 Munich to Paris
Day 79-81 Paris
Day 82 Travel to Normandy
Day 82-87 Normandy and day trip to Mont St Michel
Day 88 Train to Paris and on to London

Once we are in London, we'll have 5 days in London followed by a 12 day British Isles cruise followed by more time in England and Ireland.

I'm going to say don't bother with the Black Forest. I know I'm the Black Forest naysayer in this forum, but really, the scenery is not that impressive compared to the Alps or idk, many parts of the USA. It only makes sense to visit if you're visiting nearby areas like Alsace, Stuttgart, Heidelberg, etc. Otherwise, skip it.

I like the revised itinerary better than anything you've posted so far. I still question the wisdom of going to Santorini when you're taking a cruise that involves both Athens and other Greek islands. It just seems like a time/money sink. Also while I know people who adore Santorini, and in fairness, I have not been, my husband was just telling me about a colleague of his who visits Greece often, loves Greece, and thinks Santorini is crazy overrated. At the very least, it seems like it belongs in another trip, perhaps a more Greek-focused one. Especially since you already have a handful of outliers like Venice and Poland. Something to consider.

With as many days in Normandy that you have, I'd recommend NOT doing a day trip to Mont St Michel, but rather staying overnight. It's lovely at night, and you really get the sense of the place when you can wander around those ancient paths and the wall at night.

28 days to do Bavaria and the Black Forest sounds divine! If you want to stretch your wings a little more, go up the Rhine to Bacharach and enjoy some time there among the vineyards and life along the river. Really beautiful.