Based on this post and your other previous post, it looks like flight and ground travel costs are a concern. (You are not alone.)
"I guess it may not really matter with way we go first?"
"I'm thinking of getting a German Rail pass so we can do these trips for a more reasonable price."
Like you, I have a fairly snug travel budget. So I would say that whatever your final destinations, you need to do some hotel cost homework to conserve your €s. These costs are not static. You do not want to end up in Munich after September 20, for example, when Oktoberfest pushes rates into outer space - or when Munich is hosting a trade fair, when the same thing happens. It would be smart to plan your route around these lower prices, don't you think?
And for those destinations, rail costs will be higher if you are locked into FRA as your in-and-out airport. In addition to your Berlin.....Augsburg + Füssen travel days, you will need to add a rail pass day for the long journey to Berlin as well a rail pass day for the trip back to FRA, right? I'm counting about 7 train trips on 7 different days. Are there two of you? Were you considering the the 2nd class GRP twin pass flexi that comes in at around $470 for two? That's probably the cheapest option but still not really cheap. The more you travel, the more you pay (yes, even with a rail pass) and considering your time considerations you'll be on the train quite a bit with this plan.
"If you have must see, must do in any of these places, please share!"
This invitation for input makes me think you are not yet sure what you want to see and do in these places you have lined up. And if that's true, well, there's nothing wrong with not being sure, but I will suggest that you figure out your interests in Germany very clearly BEFORE you set up a bunch of rail trips that will usurp your time and your €s and take you to places that may or may not be of interest to you.
One good cost-saving strategy is to stick closer to your airport city, Frankfurt. You do NOT have to travel far from there to find a castle (N'stein isn't the only one - in fact it's not even a real castle) or most of the other things you'll see with your first plan. WW II history, great cathedrals, concentration camps, castles, beer gardens, walled towns with old-world architecture, interesting museums, and great scenery can all be found within a reasonable travel radius of Frankfurt.
Besides Frankfurt itself, you might include the Mosel River Valley (Trier, Cochem, Burg Eltz Castle, etc.) and the Rhine (Cologne, Rheinfels Castle, Bacharach and other old-world wine towns) as well as cities like Mainz, Worms, Heidelberg and Speyer. You can get around to most of these places inexpensively on local day passes purchased as you go. You might scratch the surface in this region with 10 days. Spend a final night in Mainz or Frankfurt, and you are only minutes from FRA airport for your flight out the next morning.
Another reasonable option might be to spend half your time in the Frankfurt area (seeing less there of course) and the other half either in Bavaria, or in Northern Germany.
To further control lodging costs, consider spending some time in smaller towns, and stay in one B&B or apartment for several nights at a time. You'll often get a price break that way. As an example... this B&B/winery in Cochem (not far from Burg Eltz) has doubles with breakfast buffet for €56/night with a 4-night stay. If you choose your base town well, you can do outings from there to other places on those day passes (typically €25 - 30 per day per couple or small group.)