You really should watch a lot of matches and learn as much as you can about the clubs. Over time, events will happen that will form a bond between you and a club even if you are at a distance. And I don't suggest merely watching the matches but read as much behind the scenes info that you are able and watch as much off the pitch material as you can find. This is the true organic way of becoming a fan. However, since you asked for assistance, I am happy to suggest BVB.

BVB has the best supporters of the Bundesliga. I will have words with any man that says otherwise. BVB has the best logo:

BVB has the best ground:

Of course, the Yellow Wall:

And I am not even talking about Pulisic but no one should blame you for liking BVB because of his presence on the club. You are an American, why should you not appreciate the opportunity that BVB is giving to the player and like them for it:

BVB is well managed from the board room to the technical area so if you appreciate quality, effective stewardship and accountability then BVB will please you. The club has significant resources but will always be David to Bayern Munich's Goliath.

Other possibilities include Hamburg. Hamburg is one of the most beautiful cities in Germany with high class, art, and culture. Steinway pianos are hand made in the city. Bobby Wood plies his trade at the club and becoming a fan is a fine way of showing appreciation. The club itself has not always had the fortune of competent management at either in the board room or on the pitch. The club has to fight to win points. The team's acronym, HSV, is of course unfortunately analogous to herpes simplex virus.

Hertha Berlin is a possibility as is Ingolstadt. If you would like to support a true scrappy underdog you will do fine with the Audi Boys of Ingolstadt. Leipzig is present for your choosing although you may find it difficult to be accepted into their fold being as you are not a true lifelong fan.

Viel Glück!