Not sure what this means. If you mean "using the Rhine Cruise boats" as transportation from Cologne to the Rhine Gorge and back to Cologne, that is totally impossible
We arrive at Frankfurt Airport at 7:20am
One option was
Frankfurt Airport to Rüdesheim/Bingen
Cruise Rüdesheim/Bingen to St. Goar or Koblenz
Train to Cologne
Any reason why your Cologne days are split?
Day trips—jumping on a train and exploring a different town, takes a bit more energy. We get up early & get back late. Versus walking out of the hotel and strolling around the city, even returning to the hotel mid-day for a break if we feel like it. When possible I like to alternate these types of days.
If 1-night stays are a problem , and you're willing to alter the 4-night Cologne stay, there'd be no 1-night stays with this simple solution:
2 days/nights in a Rhine Gorge town
(train to Cologne, morning of Day 3)
2 days/nights in Cologne
Definitely going to explore this plan. My only concern is we still tend to lose part of a day—checking out of 1 hotel, moving, checking into another hotel tends to eat up a good portion of our day. The trade off for staying in 1 place 4 nights is we get 3 full un-interrupted days but I may have to travel farther for day trips. Usually 1.5 hr by car or 2hrs max by train each way is my cut off for a day trip. So if I do 4 nights in Cologne I’ll have a near 3 hr train ride to Rüdesheim. If I do 2 nights on the Rhine and 2 nights in Cologne. I have 1 less day of sight-seeing.
Side note: We have traveled extensively with our kids and so i was always playing mom, making sure everybody had everything. Thus we moved more slowly. This is our first trip with “adult-ish” very responsible daughters. We’re traveling light so hopefully we’ll be more mobile than the past.