Where to go???

I’d plug them into a map and search transport first to see what makes the most sense for travel logistics, as even with a luxury of time you want to be somewhat efficient and not waste time.

I'd keep travelling roughly south/west - Lyon, Bordeaux, Bilbao. Food, architecture, museums/galleries...........

Slovenia - Ljubliana, Bled, Soca river valley, caves, predjama.
Budapest

I’ve been to all your current thoughts except Luxembourg. I might choose based on weather and logistics. I love Sarajevo and don’t ever need to go back to Belgrade. But if you have time to add more of Romania, you might really enjoy a contrast to Bucharest (which I unexpectedly liked). I have been to Athens in July and don’t need to do that again either. 🤣 (for weather reasons). I feel like you have probably been to Budapest.

Edit: oh wait. I missed the time of year….. oops. In that case Athens would also be a good consideration.

I've been to many of the places you listed with the exception of Bucharest.

We were in Luxembourg last November and really enjoyed our visit. I'd say it's definitely worth 3 to 4 nights. There are lots of museums dedicated to the Battle of the Bulge and a General Patton museum (https://www.visitardenne.com/en/best-ardennes/discover-our-top-10/top-battle-of-ardennes). Luxembourg city, itself, is beautiful. We took a half day walking tour throughout the city. As the public transportation is free, we also took a half day to visit the Mosel wine region in Luxembourg. And then took a couple of hours to visit the American Cemetery in Luxembourg (https://www.abmc.gov/cemeteries-memorials/about-luxembourg-american-cemetery/). Lots of great food throughout the city! We used a combination of The Fork app and Google to find good, local restaurants.

Porto was such a sleeper surprise for me. We absolutely loved it! But we were there in July so weather will be quite different. I would recommend doing the Eating Europe food tour. It was fantastic and we also visited a couple of Port houses, too (not a big port fan, but it was still super interesting.) Definitely take the boat tour of the 7 bridges. We were only there for 3 nights but could have filled a whole week and cannot wait to go back.

I'll leave it to others to chime in on the rest as it's been a while since I've visited the others except Munich--but that was Oktoberfest so none of my recent advice would be useful to you. :-)

Enjoy your trip!

Suggest picking places that are on on the opposite side of Europe, to save on wasted travel time and expense.
Romania is great, also, suggest Hungary that is not far away. If you go to Bosnia, also consider Dubrovnik and Split Croatia.
Italy is wonderful, especially Tuscany and Umbria.

Love Sarajevo, not keen arly all on Belgrade.

Like geovagriffith, apparently, I like the Adriatic coast, but not in the Winter.