Thalys offer limited numbers of spaces for passholders, once sold out they do not sell any more by any means, so you won’t be able to reserve this at a station now either.

You cannot buy a 1st class reservation and upgrade to 1st class with a 2nd class ticket/pass.

You haven’t said what times you are looking for, as there are no spaces left on Thalys from Paris either that day, the best option will be to book

Marne-la-Vallée - Chessy to Lille Europe here https://travel.b-europe.com/eurail-ge/en/booking-tgv#TravelWish using pass cover number from here

Then walk from Lille Europe to Lille Flanders (about 500m) and use reservation free regional/Inter City train connections from there to Schiphol. These connctions run hourly.

You should pick an itinerary indeed via Lille. Interrail/Eurail pass p'aces are fully booked. There you have multiple options: you might be able to take a Eurostar to Rotterdam or Amsterdam, where you can travel easily by regional trains to Schiphol.

Or you get a reservation on a TGV to Brussels, where you can change for the reservation free hourly intercities direct to Schiphol. (Ordinary tickets for that TGV are also reasonably priced, probably cheaper than pass+reservation)

Or you take the regional option. Take the train to Kortrijk/Courtrai, change there to Antwerp, change in Antwerp to the hourly IC to Rotterdam- Schiphol-Amsterdam. No need for reservations on this one, but it takes the longest time.