3 days Füssen
3 days Montreux
3 days Bernard-Oberland staying in Murren
"I thought I'd follow Rick Steves' itinerary,..."
I don't know exactly which of his suggestions you are following, but keep in mind that Rick Steves' tours are ROAD journeys. This goes back to the tips you've read from Stan here about possibly misjudging train travel times and logistics because map distances look short. Also, Rick's tour destinations are chosen in part because they fit a road-travel sequence; if he were doing a similar tour using trains, he might just choose different destinations in the first place. So I'd urge you to keep this option in mind as well - you can do your own itinerary - and also that you've got to lay out the train trips for the places you think you want to see to fiind out if they are doable. These two travel legs in your plan between Füssen and Mürren will put you on 10 different trains and require 11-12 hours of train travel.
"I won't get to thoroughly immerse myself and see everything and that's not the purpose of this trip"
It's good that you get that. But with your idea of spending time in FOUR different countries and in places that are NOT necessarily easy to travel between by train, you are pretty much red-lining your travel engine.
Time allocation is off as well, if you have 3 nights in tiny Rothenburg + 3 nights in Paris! I wouldn't attempt Paris with fewer than 4 nights. Rothenburg doesn't take more than a day; BTW it makes for a weak travel base town, if you were thinking extra days there for day trips, because it's at the end of a trunk railway and requires changing trains soon after you've boarded. A Füssen base also puts you at the end of a trunk line - you can get to the Zugspitze but you'll need to be on a bus part or all of the way.
Maybe skip the Zugspitze altogether (which doesn't compare with the Berner Oberland) or use Garmisch as a base town instead of Füssen; a Füssen base requires bus travel part or all of the way.
Use this page to examine individual journey details: https://reiseauskunft.bahn.de/bin/query.exe
"I do want to see some castles, art, German history, and definitely green stuff."
Basel - right across the German border - gets a lot of acclaim for its art options. And it's right on the route to the Berner Oberland from Germany.
For Germany, with your interest in hiking, greenery and castles, the Rhine (Boppard) would appear to suit you quite well - the Black Forest too. It would streamline your trip if you spent time in these places and in Heidelberg before taking a direct route (with a stopover in Basel??) to the Bernese Oberland and the rest of Switzerland, which from your posts appears to be more important to you. Maybe 5-6 nights in Germany, and 7-8 nights in Switzerland, with 4-5 in Paris?