Late morning on a weekday, I’d expect there to be plenty of space on those IC services. Obviously nobody can 100% guarantee it won’t be very busy but it is the option I’d take for that journey. Arrive in good time at Amsterdam Centraal and you’ll be fine.

A tip for unreserved trains, people are lazy and will crowd the nearest carriages to the platform entrance, move down the train and it can often be much quieter.

Can probably do for less-and no need to go via Brussel-change Antwerpen-and have some time to admire this beautiful classic station named´ railway cathedral´.

Also note that the not known so much city of Gent=20 mins before Brugge, has the same assortment of old houses/canals etc but is much less touristy and also more of a living normal student/uni bigger town.

Book at least 7 days advance, via nsinternational, single TO there and back-then you get lower price. There is not even need to use specific train. In fact summer is more quiet on local trains-plus that even IF (the IC stops 12 mins after dep. Ams at AMS-airport-when a load of people will get off) you can find no seat-there will be after next stop, not 4 hrs. Its like 2hr to Ant, then 1 hr 20 mins to B. You can also break on the way-so may get off at G and if you do not like it-go on and compare!

And if you do not come back same day and do more longer train trips-check  3 day pass as 1country-but covers 3: Nederland/Belgium/LUxemburg.

However-do check times again 2-3 days before, as mostly weekends there may be works on the line=other timings. This happens quite often. Selamat jalan di kerata api, lah!