You're not entirely clear on which day you want to do which journey but:

• The best website to check for Thalys availability is:
  https://www.b-europe.com/EN/Booking/Pass#TravelWish
  There's however a limited number of pass holder seats; this page will only show trains with availability. Keep clicking on “Later trains” to see when there's availability.
• Availability on Brussels-Paris is often better than on Amsterdam-Paris. You can use the reservation-free intercity train (runs each hour) to connect from/to Amsterdam.
• From Paris to Bruges, travel via Lille shouldn't take much more time and is cheaper anyway. Look for a TGV Paris-Lille and then you can travel reservation-free to Bruges (change at Kortrijk).
• If you don't even find available seats between Brussels and Paris for a journey from Amsterdam, then also travel via Lille. Reservation-free Amsterdam to Lille (change at Antwerp) and a TGV Lille-Paris.

To add a fully seat reservation free option to get from Paris-Nord to Brussels

• There’s also an option to go to Brussels or Amsterdam via TER to Maubeuge. Change there for a train to Charleroi, from which you have frequent connections to Brussels, bit slower than over Lille. If you want to go directly to Bruges from Paris to Bruges you better stick to the Lille option. (Paris-Lille-Kortrijk-Brugge)

The only availability between Paris and Brussels on 5th July is 17:25 - 18:47 and 20:22 - 21:44 for 27€. You better go via Lille-Flandres as advised : not much longer and many seats available for 10€.

Do not pay 27€ extra for Thalys between Brussels/Antwerp and Amsterdam : really not worth the price (most trains are full anyway). Instead take the hourly reservation-free IC to cross the border, you'll have more flexibility too !

Thank you for your replies. I already have bought a global pass on Eurail but cannot see any seats for July 6th from Bruges to Paris. Same for July 12 or 13 from Amsterdam or Rotterdam to Paris. Leaving Paris seems to be easier than coming back. It is so frustrating. I wish I had not bought the global pass.

Have you checked Paris-Lille as per our suggestions? You don't need reservations for the connecting trains from/to Lille.

There is availability on 12th and 13th July between Amsterdam and Paris but not on all trains. (32€ seat reservation)

- 12th July : 17:11, 18:11, 19:15

- 13th July : 06:11

If you'd like more flexibility, book Brussels - Paris (27€) and take the reservation-free hourly IC train as we advised. Or consider the route via Lille.

Thank you, I’m getting trains to Lille. The only question is, if I have a reserved seat, can I use it on any train or will it only work for the train I reserved it for?

Only for the specific train you reserved (that counts for every kind reservation, even the optional ones!)