Munich or Nuremberg?

If you don't already have lodging of Octoberfest in Munich you need to start looking. It will be expensive. I like both towns, visit at least once a year, but would suggest you consider someplace more in line with Venice-Berlin. Like Prague, or Vienna. Or, even better, would be to fly to Poland. Ryanair will get you into Wroclaw from Venice for less than $50 and a really good hotel will run about $100/night.

Nothing against nostalgia but how about Prague and / or Dresden on the way?

I'll be in Prague and Dresdan after Berlin. However, I have wanted to go to Poland and haven't had the chance. That's something to look into for sure.

(Yes, the hotels around Munich are very expensive around Oktoberfest. That's one reason I'd only stay 2 nights.)

Life is short. It's cool when you can find pleasure and beauty. If you have grand memories of Munich and Nuremberg and it adds to the beauty of your life, re-visit those places -- don't be distracted by places thrown out by people who know absolutely nothing about you. Trust your instincts. I think 2 nights in Munich and 2 nights in Nuremberg are fine. I like to minimize moves (and I have no particular attachment to Munich or Nuremberg), so I might be inclined to do Nuremberg as a day trip to minimize a move, but I also understand the cost issue. Four nights in Nuremberg with a day trip to Munich/Oktoberfest might work, too, and be cheaper?

On your way to Berlin, consider a stop at Erfurt if you haven't been there. There is a large locker area at the train station to store bags if you want to walk around and check out the place. It is a quite attractive city with a large half-timbered old town that largely escaped Allied bombs during WWII. It has an Oktoberfest of its own -- far smaller than Munich's -- that I found quite enjoyable. https://www.erfurt-tourismus.de/veranstaltungen/hoehepunkte/erfurter-oktoberfest (the main Oktoberfest website is linked on that page but right now covers the upcoming Spring Fest).

Thanks for the suggestion on Erfurt. That's a good one. It looks great! I'll check into the train schedules.

The ICE trains go Munich to Nuremberg to Erfurt to Berlin, so train schedules should be easy.

That's super.

A nice Deutsche Welle video on Erfurt: https://www.youtube.com/watch?v=KppLtPseAME

If you decide to visit Erfurt, you might want to consider staying there for a night. Not a fan of Frankfurt, we landed in Frankfurt and immediately took the train to Erfurt on our way to Berlin. I had not been there before, at least not to stop and visit.
We enjoyed a nice meal and walk about, and the next morning toured the local museum where I learned that my knowledge of the mistreatment of Jews was horribly limited. Erfurt is worthy of a stop.

Dave, thanks for the video link. It was great. You all are giving me super ideas.

If you were to stay in Erfurt, I can highly recommend this small, mother/son-owned hotel located almost on Domplatz. It's where I stayed in October 2019 when I went to Oktoberfest, which is located on Domplatz. I went back to my hotel late one evening and did some work at the desk in my room as the singing from the beer tent wafted through the open window.

https://www.hotel-domizil-erfurt.de/startseite.html

Thanks. That looks great.