ICE vs EC trains
 

We are traveling from Brussels to Salzburg in late April, 2024. Bahn.de offers option #1 with an ICE train Brussels to Frankfurt, then an EC train from Frankfurt to Salzburg. I like this because it's fewer switching trains, only switching in Frankfurt with a 49 minute interchange.

Option #2 is ICE trains Brussels to Frankfurt, Frankfurt to Munich, then a RJX to Salzburg. I'm a little nervous about the tight connections/interchange times of Option #2, with only 22 min in Frankfurt, and 17 min. in Munich. Can you please offer advice of one option or the other and reasons why?

Also, any advice on choosing seats on these trains... Thank you.

I would have a look at Seat61 website. Comprehensive information on everything train related.

We prefer the Railjets (RJX) to EuroCity trains. OTOH, having as few changes as possible is usually a good approach. In 2023, I made numerous train trips (for business mostly) from Switzerland to Germany -- in particular, to Munich, Frankfurt, Mannheim, Freiburg, Baden-Baden, Penzberg, Ulm, and Stuttgart. On the German side, delays and cancellations were the norm rather than the exception, so I would suggest having nothing less than 10 minutes between connections; the 22 and 17 mins you've seen sound ok. I also took the Railjet between Zurich and Vienna and that was a lovely ride. Be advised that there is major construction work going on at Munich HB (at least there was in late October). FWIW, would also recommend first class if budget allows. We always go first class and book seats in the quiet car if available.