Brussels to St Goar

Looking ahead a couple of months I see Sparpreis fares in the early morning (6:25) of 19 EUR per/p.
http://reiseauskunft.bahn.de/bin/query.exe/en?country=USA
Buy up to 92 days in advance to get these low nonrefundable prices. Print tickets at home.

Confused with Linder/Bahn cards.....

Do you mean Länder-Tickets? Länder-Tickets are for regional trains travel in a single state (Land) in Germany, not for travel outside of Germany. You'd have to go through several German states (Nordrhein-Westfalen and Rheinland-Pfalz) to get to St. Goar, and it probably wouldn't be cost-effective to buy multiple Länder-Tickets.

There are also regional passes for all of Germany (Schönes-Wochenende on weekends and Quer-durchs-Land on workdays). These, also, would not be time or cost effective compared with the SparPreis Europe tickets mentioned above.

A Bahn Card is a card giving 25%, 50%, and 100% discounts on tickets, but the cards are expensive and only worthwhile for people doing a lot of travel.

If you get a SparPreis ticket, it will probably use an ICE from Brussels to Köln, an IC or RE from Köln to Koblenz, and a regional train (MRB) to St. Goar. For SparPreis tickets, the high speed trains (ICE/IC/EC) are specific to the ticket, i.e., you have to take the train shown on the ticket, but the regional trains (RE, RB, MRB) are open; you can take any regional train on the same route that day. So if you arrange it for an RE from Köln to Koblenz (you might have to manipulate the Bahn website to do this), you can spend any amount of time sightseeing in Köln before going on to St. Goar.