advice for upcoming Germany trip

Here's a good source for train information:
https://www.seat61.com/train-travel-in-germany.htm

German Rail passes can be a real convenience for first-time travelers to Germany. For long train journeys they will save you money over tickets bought at the counter on the day of travel and they can make good sense overall if you have say 4 or more long-distance journeys in mind but don't wish to travel on a strict schedule.

Most of us more experienced travelers tend to pre-purchase online "saver fares" for those long trips instead for the purpose of saving some Euros. I normally buy mine 2-3 months in advance or earlier for the best prices. For you, the pass looks like a very good choice since prices for your journeys just 2 weeks out are very likely to be much higher at this point than they were 2-3 months ago. I strongly suggest you buy whatever rail pass you decide on in GERMANY at one of the rail aid stations - probably at your fly-in airport.

https://assets.static-bahn.de/dam/jcr:ad222c00-70a1-4159-95c6-3fbd25af917a/240076-319159.pdf

The agent will advise you on the use of the card and might also help you decide how many days you will need based on your itinerary.

Once you have a more precise itinerary in mind, you could post it here for help with that decision as well.

If your itinerary includes large cities like Berlin, Munich, and others, you will need to supplement the rail pass with local day passes for public transport systems there (the subways, trams and buses you will need and are not covered by the GRP.)

Cologne is not a great base town for visiting the popular Middle Rhine Valley. I suggest you see about a room in Boppard for a stay there if you're interested in the MRV. Most people who have Cologne on their lists are looking to spend some time in the MRV seeing the old-world towns and castles, hiking, river cruising...

https://whc.unesco.org/en/list/1066/gallery/
https://whc.unesco.org/en/list/1066/

The town of Boppard offers completely free rail transport for local outings to all its guests - your day trips by train to Marksburg Castle, Burg Eltz Castle, Rüdesheim, St. Goar, Cochem and other nearby towns would not have to use up any of your rail pass days. Read about the Guest Ticket here.

I left Germany two weeks ago. Your CDC card works everywhere. Since Americans are not in the German database, you will not get a health pass at a pharmacy. It is not necessary. Bring a KN95 or N95 mask though.
Enjoy your trip. Your original vaccination of Moderna plus a Moderna booster is just fine.

Fantastic going to Germany for the first time as a solo traveler and for a month too ! Totally nothing to be nervous about, certainly not going over alone. Most of my traveling in Germany over the years was done solo.

I would suggest getting the 15 day Pass, if you plan on possibly taking night trains too, say from Hamburg to Munich.

My suggestion is to include towns and cities in North Germany as well as places in eastern Germany, eg, Leipzig, Weimar, Dresden, Potsdam, Jena, Meissen, etc. in your itinerary.

Hallo! We are in Berlin right now. We have a German rail pass for a set number on nonconsecutive days, which we purchased at the Frankfurt airport at the Reisezentrum ( the DB office). After only three trips it is clear we will break even/ save money.
The lesson of Covid we learned is to avoid non refundable travel. With the rail pass we can be spontaneous and react in real time to things like…weather. It has been very cold, rainy/ snowy, windy… and that has caused us to adjust our activities for day trips more focused on the indoors.
Our CDC cards are fine, and many restaurants didn’t even want to see them. Mask usage is almost 100% on transport, in train stations, inside stores, and at events. At the opera in Dresden you had to wear a mask unless you were in your seat. In Berlin you had to wear your mask for the entire concert. And there was a guy walking around enforcing that. Germany has several states/ local jurisdictions that clearly have autonomy in their Covid responses.
We brought masks, but immediately purchased some FFP2 masks at a drugstore at the airport upon arrival to make sure we had the type required. ( Cheap and readily available everywhere, they are actually the KN95 type that go behind your ears instead of across your head. )
Covid? No problem. The weather on the other hand has been a real challenge. Bring cold weather options. Umbrellas are useless in this gusty wind, but a waterproof raincoat with a hood is invaluable. Spring is a volatile time in many places, and Germany is no exception. 🥴
Other than the diligent masking and having your CDC card handy, it is almost Iike normal. I felt more worried about my health in the US than I do here.
Send me a message if you want any other real time information. You will have a wonderful time!
Germany was our first entry into international travel as well. Unlike us crazy Americans, Germans value language learning. So you will always be able to find English speakers who can help you.
Do not stress. Anticipate eagerly!

In case you are interested in more than just the scenic Middle Rhine:

https://community.ricksteves.com/travel-forum/germany/can-i-see-enough-of-cologne

I personally found Munich over-rated. You didn't mention Leipzig or Dresden. But we don't know your personal interests, like art, WW II, or what.

As long as OP stays within Germany, they don't need reservations for trains. The only trains that need reservations are those run by foreign train companies across the border (like the TGV to Belgium or France or the PKP trains to Poland).

That said, I agree with your overall assessment that a rail pass might be more expensive for OP than individual tickets. Since rail travel is still lower than usual due to the pandemic, it's usually possible to get good limited deals via the app even if you book only 2-3 days in advance, especially if you're flexible enough to travel out of rush hour times.

https://assets.static-bahn.de/dam/jcr:ad222c00-70a1-4159-95c6-3fbd25af917a/240076-319159.pdf

https://whc.unesco.org/en/list/1066/ shaglevoojio

Thanks for sharing links.