Train from Dusseldorf to Amsterdam

You will save some money by buying Super Sparpreis Tickets. They are train specific and nonrefundable.
There is a connection every 2 hours with a regional train to Duisburg then transfer to a direct ICE train to Amsterdam Centraal. You are allowed to take an earlier regional train to Duisburg, but you must travel on the specific ICE train from Duiburg to Amsterdam shown on your ticket.

Obviously, check the train schedule for your actual travel date.

Information on Duesseldorf Airport:
Here is a map of the airport terminal and train station which is in the far right hand part of the map.
https://www.dus.com/-/media/dus/reisende/anreisen/bus_bahn/skytrainplan.ashx

The airport is connected to the train station by a monorail called the Skytrain. Rental cars are returned in a parking garage, shown to the right of the terminal. No experience here, but I imagine there is shuttle service back to the terminal where you can get the monorail. Otherwise there is a skywalk to the terminal. The SkyTrain is not free, it is VRR ticket you can buy out of a vending machine at the SkyTrain stations. It is 3.60 EUR per person.

There is a regional train to Duisburg about every 10 minutes and you can hang out there waiting for your train to Amsterdam.

so you’re sure to get a seat at a reasonable fare.

You don't get a seat automatically, you get a ticket for a route. If you want to reserve a seat you can do that for a small fee.

We did the reverse ICE train trip and bought our tickets well in advance when our plans were solidified. The longer you wait, the higher the price goes.