Booking seats or showing up?

The pass is your ticket. You are looking at booking reservations.

Without any details such as dates and departure times, it is almost impossible to advise. It's also still quite early. bahn.com will show how busy trains in Germany will be, but only a few weeks in advance. Also, because of engineering works, it's best not to book more than 3 weeks in advance anyway.

For Salzburg to Vienna, check on tickets.oebb.at how busy your train will be, also a few weeks in advance.

Thank you for advice! Here’s more info on our schedule in case it’s helpful

3 people Munich airport → Nuremburg on 22/12/26 (our flight lands at 9:00 but don’t know what time train we’d want, so advice here would also be helpful)

4 people Nuremburg → Munich 24/12/26 around 15:00

6 people Munich → Salzbourg 28/12/26 around 10:00

6 people Salzbourg → Vienna 30/12/26 around 14:00

And (sorry to add on so many questions), how does the pricing change as we get closer? Is it bad to wait until those few weeks before?

Reservation costs stay the same (5,50€ from DB directly, even if the journey involves transfers).

For normal long distance tickets its best to book early as the price increases.

From Munich Airport to Nuremberg, there are frequent departures, many of which are on regional trains which you don't need to (and can't) reserve.

For this journey, just turn up and take the first connection available. 

Nuremberg-Munich on Christmas Eve, I would reserve.

Munich-Salzburg and then Vienna - probably worth it, since there are 6 of you wanting to sit together. But if you don't mind risking being split up, save your money and spend it in the restaurant car.