Should I get a rail pass or just buy tickets as I go?

It is highly unlikely a rail pass will be the most economical way to go. Bought in advance I found tickets for the longer stretches of your trip for an ad random date mid-June to be not more expensive than some EUR 25 (in 2nd class which is fine enough for most people).
The first leg of your trip from Schiphol airport to Brussels is best done with the Intercity Brussel. The Thalys service is slightly faster (around 45 mins), but comes with a compulsory reservation for a specific train. Tickets for the Intercity are valid for all trains running that day, which makes it a lot safer with all the uncertainties with the arrival of your flight and the handling at the airport.
Buy tickets for trains to, in and from Germany at the Deutsche Bahn. Tickets for the Intercity Amsterdam-Brussels can be bought at the NS (Dutch railways) or NMBS (Belgian railways). Loco2 and Trainline are also reputable agents, with Trainline possibly slightly more expensive.

The train planner of the Deutsche Bahn gives the best overview of all connections, also for those outside Germany.

Our host covers this question in his Travel Tips, top left blue menu. Especially in Belgium and Germany, a railpass is a horrible waste of money for daily daytrips that cost less than $15 a day. Unfortunately, you are probably too late to get single-ticket discounts for your long-distance jumps. These kind of (non-changeable, non-refundable) rail tickets are now priced like airline tickets. So you have to be willing to lock yourself in, as well as buying well in advance.

You might be able to save money (if you care ... ) by taking slower trains for the longer distances. You should start browsing the Netherlands and Belgian and German rail websites (not a third-party reseller) immediately. You have a lot of mouse-clicking to do.

You might also use the Search box to learn about regional day and weekend discount tickets in Germany, often posted in answer to specific questions by Russ. On the Rhine, we just bought tickets at (unattended) stations as we went. But you need smaller bills to do that.

Offhand, I'd say no to the Eurain Benelux pass, but it depends. IC trains in Belgium (what you'll be taking for the most part) are pretty inexpensive. Luxembourg to Koblenz is by regional train, also fairly inexpensive. I suppose you could take IC/ICE train from Koblenz to Emmerich, but you can buy advance Sparpreis tickets for that.

Now going immediately from Amsterdam Schipol to Brussels can be done with the high speed Thalys, but it is pretty expensive spur of the moment, and you really need that because you can't be sure your plane will arrive on time. Cheap tickets are nonrefundable and have to be bought well in advance. Even with a rail pass, you would have to buy seat reservations at 15 EUR per person. The Thalys takes 1 1/2 hours to get Brussels. An alternate is take the IC Direct train that takes 2 1/2 hours to get to Brussels.

In short if you want to take the Thalys, then get the 3-day 2-person Eurail Benelux pass for $274 and use it for the trip to Brussels, the r/t to Bruges, and the trip to Luxembourg, then pay the Ghent and Antwerp trips out of pocket. If you want to take the IC Direct train to Brussels, just buy point-to-point tickets. That will be your lowest cost plan.

I don't know if you are Seniors, but if you are - the Belgium trains all can get very cheap indeed.

Over 65 (your passports will likely be checked for proof) if you travel after 9:00 in the week, you can each go for €6.50 return (round trip) anywhere in Belgium.

The Seniors ticket is valid year round: Monday to Friday: from 9:00
No timetable restrictions: Saturday, Sunday and if the public holiday is on a weekday.
Except: From 14 July until 19 August included: not valid on Saturday and Sunday

Not valid on 15 August.

All the details at:- https://www.belgianrail.be/en/tickets-railcards/age/adults-seniors/one-time/seniors-ticket.aspx