Brussels Airport to Amsterdam

You only go through immigration and customs at the airport, on the way to Amsterdam.
As long as you do not buy tickets from the high speed rail service Thalys you don't have to buy tickets in advance. You can wait until you get there. The prices on Belgium and Netherlands rail services from Brussels to Amsterdam (except on Thalys) are fixed and the ticket is good for the who!e day as the seating is first come first serve
https://belgianrail.be/en

If you opt to buy tickets in advance and the service is through Thalys, give yourself 3 to 4 hours between your arrival in Brussels and your departure to Amsterdam to catch your train
For example arriving in Brussels at 9:20 departure to Amsterdam 12:20

An easier alternative is to take the Intercity service to Amsterdam: it takes some 45 mins longer, but it has no compulsory reservation. A ticket for that train is valid on all Intercity trains running that day. No nerve wracking in case your flight is late or the airport procedures take longer than expected. No useless hanging around because you had a safe time for the Thalys.

There are hourly Inter City trains from Brussels Airport to Amsterdam. You can get there slightly faster by travelling to Brussels Midi station in the city centre and getting the high-speed Thalys train, but it only saves twenty minutes so isn't really worth the hassle.

The intercity from the airport to Amsterdam has a 5 min transfer in Rotterdam. Is that enough time? Thank you for your help!

It does not matter you can just get on the next train to Amsterdam from Rotterdam. The ticket is good for any train except Thalys. There are a lot of trains passing from Rotterdam to Amsterdam

The Benelux Intercity from Brussels Airport to Amsterdam Centraal is direct. No need to change anywhere. Look at the route here: https://www.nsinternational.nl/en/trains/intercity-brussels

For the most part, the hourly ICD (leaves at 11 past the hour) trains are direct with a few exceptions. The 11:11 still requires a connection at Rotterdam, but you arrive on track 12 and the train to Amsterdam Centraal leaves from track 11. They are on the same platform, so you just exit the train and walk 20 ft across the platform and board the waiting train. 5 minutes is easy.

... and otherwise you wait 15 mins for the next train to Amsterdam. In this part of the Netherlands the trains run like a metro system.

Thank you all so much. This has been incredibly helpful.