how to get to Delft from Paris

I’m not sure where you found this information, but there are no direct trains from Brussels to Delft. To get from Brussels to Delft you would always need to change trains in Rotterdam. So why not get the Eurostar from Paris to Rotterdam and change there to the train to Delft.
Buy your tickets on the official website for international travel of the Dutch National railway company https://www.nsinternational.com/

Basically you got it. From Paris Gare du Nord, the Eurostar is the most obvious option, though Ouigo is the budget option. Where you switch from Eurostar to local trains is up to you, I would be tempted just to go all the way to Rotterdam, then catch one of the frequent local trains to Delft, about 15 minutes. However, you may have more options by changing to the Intercity Direct trains in Brussels. For example, if you want Ouigo due to the cost, then you will have to change in Brussels.

For an all in one ticket, you likely would need to book on NS International https://www.nsinternational.com/en, they sell the Eurostar leg as well as all other legs.

For the main leg, look at Eurostar https://www.eurostar.com/us-en and for cheap tickets on Ouigo, https://ventes.ouigo.com/en-GB

If you change in Brussels, then book Brussels to Delft on SNCB International https://www.b-europe.com/EN/

Thank you both for the advice and for taking the time to post the helpful links. We’ll be taking the Paris to Rotterdam route then train it to Delft. Happy travels.

Good to know: Within the Netherlands the trains function pretty much as a subway. There are 10(!) trains per hour from Rotterdam to Delft. When you arrive in Rotterdam you can just take any one of those...

Really good to know that trains from Rotterdam to Delft run that often. I noticed that the NS tickets I purchased didn't specify a particular time so I'm relieved that we can just hop on the first train that's available. Many thanks.