I would simply visit Regensburg enroute to Nuremberg. Drop bags in a Regensburg station locker. Spend the whole day in Regensburg, 3 nights in Nuremberg. That gives you 1 day in Nuremberg plus 1 day in... gosh, so many nice places nearby for day trips... Bamberg, Bad Windsheim, Rothenburg, Iphofen, Würzburg...
Do you really need FOUR nights in Munich? I'd rather have at least 4 nights in Nuremberg so I could do a second day trip from there.
I'd use the Bayern Ticket on the regional trains for Munich > Regensburg > Nuremberg . It's valid all day (weekdays, after 9 am.)
For Nuremberg > daytrip town use the VGN Tagesticket Plus (valid all day, every day.) Cheaper than the Bayern Ticket and good for the day trip destinations I mentioned from Nuremburg (Except Würzburg; use the Bayern Ticket instead on that day.) One of these tickets covers your whole day of travel, just like the Bayern Ticket.
Then use the Bayern Ticket on the day you do Nuremberg > Munich.
NO advance booking or payment. NO reservations. Ride the regional trains (no ICE trains.) Do what the Germans do and buy these tickets on the day of travel, just like you would buy a municipal bus ticket.
The alternative to day-by-day tickets is the Deutschlandticket (subscription, €49, good for a month, covers all your trips by regional train.)