Frankfurt, Landstuhl, Munich and Hamelin

Best would be to plan the reunion date, place, etc and build out the rest from there logistically so you’re not back tracking or trying to wedge things in that don’t fit if this reunion is a priority for you.

Seems like the dates you will be travelling and for the reunion are needed before you can plan the trip ... once those are set, you can plan the best route for all the different locations ... are you renting a car? ... travelling by train? ... these also need to be determined so you can plan ... a car gives you greater flexibility but the cost will be higher than trains (and you have to drive instead of sitting back and relaxing) ... I'd be happy to help, just need some more info before I can provide some thoughts.

Do you know the term "Open-Jaw" flight, or Multi-City, as it's called on airline websites? You may avoid backtracking, at no extra cost. What time of year? How many guests contemplated?

I personally would not combine Hamelin and Munich on the same trip, too far apart. But you have multiple personal objectives. You didn't say how long the total trip is. You might give some thought to how much more traffic there is on the highways than 60 years ago. And how little there is to see from an average Autobahn. It might be useful to ask or imagine how the guests prefer to travel to the reunion, which might possibly favor a rail hub, to minimize train changes. I only say that because most Germans know how good their rail network is, not because I have any German relatives!

Although Frankfurt is a major business and conference center, it may not be the most picturesque place in Germany. (opinion.) I'm also not a big fan of Munich, but Düsseldorf has a pretty big airport, too.

"We also plan to have a central location (not determined yet) for all other German cousins to come to for a familie reunion. What is the best way to plan this?"

How many people, how long would this event be, what time of year, where do the others reside... too many open questions for any of us to respond. Assuming that your German cousins live in Germany, wouldn't this reunion be simpler to plan (and more likely to succeed) if one or more of the German cousins were to organize it?

As for the Munich > Hameln journey, I wouldn't be concerned about the distance... it can be done in just 6 hours by train with only one change of train. Spend most of the day in Munich, leave at around 3 pm, have dinner on the ICE train or picnic in your seats, arrive around 9 pm. No sweat.

If you're in south Germany, getting to Hameln is straight forward. I would suggest taking the train from Munich to Hannover, which is what I would do....easy and direct.

Unless you want to see Hannover, some sites are worth it, I suggest staying in a smaller town such as Minden an der Weser, stayed there a couple of times. I know Hameln only because of doing an afternoon trip there from Soest/Westf. but never stayed in Hameln.

Frankfurt am Main for sure is the most central traffic hub in Germany by train, road and plane.
Thus, I would plan your family reunion close to Frankfurt, maybe in one of the nice towns in Taunus area like Oberursel, Bad Homburg or Koenigstein.
If you fly in and out of Frankfurt it would be most efficient to do a roundtrip by train
Frankfurt – Landstuhl – Munich – Hamelin – Frankfurt

How many days do you plan to stay in Germany? Then you can judge if you feel well with 4 travel-days between locations.

If you plan to go to Munich just for the BMW museum, you husband might love also the following:
https://www.deutsches-museum.de/en/verkehrszentrum/exhibitions/urban-transport

Thank you for all your responses ... very helpful. We plan to travel from June 7th to 22nd 2023. These are early stages of checking flights out and the best airport to fly into.