score:3
Just like when you have a direct Amsterdam to London Eurostar train, the check in closes 30 minutes before the train leaving Brussels.
The Thalys trains and Eurostar trains use the same part of the station so walking time should not be that long.
I recently did this journey the other way round and it is doable, but you will not have any spare time at Brussels. If your Thalys train is running late, speak to train staff for advice.
Last week they were working on the connections between the platforms and with the Eurostar check in you need one particular entry anyway, so your position in the train can make it easier or harder.
In Amsterdam you may have to show your ticket getting on the train but that is all, no special checks. But 15 minutes does not leave you any margin if you miss your transport to the station. I would plan to arrive earlier, I mostly arrive at least one train earlier in Amsterdam Centraal.
Check-in times for Eurostar trains, what is the actual minimum time? for the actual closing times for the gates.
I recently went direct from Amsterdam to London and with a change of trains on the way back in Brussels, so I did not have to check in at Brussels, just like you do not need to check in at Amsterdam.