Benelux Countries in April 2022

I am doing the same trip at the same time, except for 16 days! I think the 4 days you will be there is enough time in Amsterdam. Were you going to see the tulips at Keukenhof? The Dutch and Belgian trains aren't as cheap as, say, Italy, but you'll have to decide if the Benelux Pass is a cost effective option. I will be moving around around quite a bit and even though I won't 'use' the entire value of the pass, the convenience of not having to buy tickets for every.single.trip is worth something to me. Would love to touch base as we get closer to see if maybe we could go out for rijsttafel or something.

If there is one thing you want to do is pack light. Make sure you can haul your luggage up and down stairs if needed.
DB shows direct trains from Amsterdam Schiphol airport to the Brussels Midi station that takes 1h 45m. You may want to sleep in Brussels near or on the Grand Place Square or continue on to Bruges (1h by direct train). Brussels will be easiest but Bruges will be prettier. Some people will suggest sleeping in Ghent (30-minutes). If you sleep in Brussels, take a day trip to Bruges.
From the Brussels Midi station you can take a direct train to Luxembourg (3h 30m). To get back to Amsterdam, you’ll need to transfer at the Brussels Midi station taking (6h 30m).
Tickets tend to be cheaper when traveling point to point instead of buying a rail pass. You can learn more on Rick Steves website: https://www.ricksteves.com/travel-tips/transportation/trains. Rail Europe is also helpful but their point to point tickets tend to be pricier than other websites: https://www.eurail.com/en/eurail-passes. Good information on rail passes though.

there was a comment above that Dutch and Belgian trains are not as inexpensive as in Italy...

Over 65s can go out and back anywhere in Belgium for a set price of €7.20 - that's for both out and back.

https://www.belgiantrain.be/en/tickets-and-railcards/senior-ticket

These Senior tickets easy to get, no pass required.

Also, for Amsterdam to Antwerp and Brussels, the fast "Thalys" train can get expensive but there are cheaper Intercity trains that take barely 1 hour longer and do not require booking as far ahead (1 week is enough to get good fares).

Note that this discussion is only the beginning of your research. It's not a difficult itinerary, but there are a lot of details to learn about. The search box top center of this website is good way to start. For example, the choice (usually, NO) for a railpass is related to the search terms "base in Belgium" or "Flanders cities to visit". If the railpass is limited as to days of travel, you don't want to use days up for two 45 minute rides because you are staying in (say) Antwerp while visiting Brussels, Bruges, and Gent by daytrips.

Likewise, the Brussels Midi station is not very near Grand Place, but the Centraal station is. Most tickets for trains that only stop at Midi include the right to one free rail ride to the other two stations in Brussels, but you need to make sure of that. (Or, pay for the subway below the station.) Lots of less "special" trains stop at all three stations all the time.

Of course, Covid rules between contiguous countries are unknown six months in advance!

https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-besides-bruges-and-ghent

https://community.ricksteves.com/travel-forum/netherlands/amsterdam-to-brussels-to-brugge-back-to-amsterdam-by-train

https://community.ricksteves.com/travel-forum/netherlands/day-trips-from-delft

If you wanted to go to the Keukenhof Gardens outside Lisse to see the tulips, the timed tickets are now available for purchase.

Thank you one and all. Yes this is the beginning of our planning and yes COVID can change everything. We have now removed Luxembourg from our plans. We'll concentrate on the Netherlands and Belgium. Everyone has been most helpful.