Should we buy train tickets in advance for RT day trip from Amsterdam to Brugges?

If you choose a route on the higher priced faster train operated by Thalys requiring a change of trains in either Antwerp or Brussels than you book that in advance.
https://www.nsinternational.nl/en

The price will go up on Thalys but not on the IC trains; but a route using Thalys is faster.
I personally wouldn't do this as a day trip since it takes 3 hours each way more or less.

Economy class is fine, First class has more room, but this is not a good idea for a day trip, 6 hrs round trip. Stay overnight in Brugges.

I'd agree: Bruges is much too far for a day trip, and deserves more time than you could give it.

Why the whole way to Brugge while there are so many worthwhile places to visit within easy reach of Amsterdam? Go to lovely Enkhuizen, one hour by train from Amsterdam, with a wonderful open-air museum? Or the perfectly preserved fortified town of Naarden, even closer? Or, weather permitting, the unique windmill complex of Kinderdijk, easily combined with underrated Dordrecht?
Dozens of possibilities without the hassle - and the cost - of the long trip to Brugge?

Don't go to Bruges on a day trip from Amsterdam. Spend at least two nights and then all that train travel time will be worth it.

Agree with the others--don't go to Bruges on a day-trip from Amsterdam. There are many, many worthwhile trips from Amsterdam that won't take as long and are just as rewarding. For example, Haarlem is only 15 minutes away. A trip to Hoorn for its great museum and Enkhuizen for the spectacular open-air museum make a great day trip. Or consider a visit to Delft and The Hague--something old and something new. Buy Rick's book and consider all the possibilities...and save Bruges for another trip.

"Buy Rick's book and consider all the possibilities...and save Bruges for another trip."

Yes, Rick's Amsterdam & the Netherlands book has all kinds of day trips from Amsterdam, complete with detailed descriptions and foolproof transit directions. You have your pick of cities, small towns, and more rural areas - all much closer than Bruges.