Moin, Moin! As a West Coast expat living just outside of Bremen, I would say that your three solo day-outings look good. The three museums you mentioned in Bremerhaven are all good-quality and varied; just be sure that they are open on Mondays in the slower winter season.
Though Heligoland is nice in sunny and dry weather, I fear that you aren’t going to get much of that in March (it snowed earlier today but didn’t stick). I’m not sure what there is to do on most of the German North Sea Islands when the weather is bad but a quick google search showed that there is a museum there at least.
Bremen is very doable in a day. I would recommend booking an Beck’s Brewery tour as well as a guided tour of the UNESCO town hall. I believe there are daily English tours for both but I might be mistaken. Best booked in advance and you can reserve/pick up the tickets through the TI. I also recommend going to the Schnoor Quarter, as it is cute to meander through, and walking the Boettcherstrasse. You can catch the porcelain bells play on the hour and watch a ‘story’ on the rotating turret near the end of the alley. Both are small areas that take maybe 10-30 minutes to go through, but if you’re in Bremen anyways you might as well go to them as the sights are all basically around the downtown square. There is a kitschy pirate ship docked near the end of the Boettcherstrasse that serves various flat pancakes but don’t be lured in. It’s good for kids but that’s about it. If you fancy it, there is a permanent variety show group in Bremen that does shows nearly every night at the GOP Variete next to the Steigenburger Hotel. It is a fun way to spend an evening and isn’t too costly, but it definitely isn’t a very Rick Steve’s-ish type place.
In Hamburg I would recommend a harbor tour if they are running, and hiking to the top of the spire at St Michael’s church. The Elbtunnel is neat because of the concept but I didn’t find it very interesting to just walk through a tunnel and back. I prefer to walk through the Hafen City and look at the mix of architecture. And yes, people really seem to enjoy Miniatur Wunderland—you can get a ticket online and view when the busy entry times are. I can’t comment on the Elbeharmonie as I haven’t had the chance to attend, and there are also a few theatres that have musicals (like Aladdin and Lion King…in German). Hamburg is also known for having the more expensive brands luxury shops in the region.
I personally take guests to Hamburg and we can cover most everything in a day, especially when the weather isn’t great for strolling along the Ausser/Binnenalster and we don’t have anyone searching for Chanel and Gucci.
I would suggest looking up information on Lübeck, famous for its marzipan, as well. I haven’t been there but I have heard good things so maybe it is worth considering.
Happy Travels!