Brussels to France

I'd probably rent the car in Versailles. What are your plans with the car? Are you actually going to spend time in the palace and gardens or is this just the starting point you've picked?

Yeah, a car is totally unnecessary for visiting Versailles. Have you considered taking the train to the first non-Paris destination? Or have you already determined, by research, that the rental will be much cheaper with pickup and return at the same place? It may cost gasoline and tolls to get to the first distant destination.

You get the best advice here if you give the month of travel, number of travelers, and a rough itinerary. For example, are you continuing in the direction of Versailles with the car? Where are you sleeping the night you arrive at 6PM? Lots of France is quite accessible by train, and few downtown hotels have garages of their own. Make sure what best serves your actual needs, rather than making assumptions about the freedom of a mighty steel steed.

I think Thalys has compulsory reservations. Although advance purchase is much cheaper, there is some risk that your flight (or immigration lines) might make you late for the train you want to Paris. I have personally (pre-pandemic) waited in some miserable "All Other Passports" lines at BRU.

Hi!
When you say "arrive at 6 PM", do you land at 6?
If so, I would hesitate to plan for a Thalys ride to Paris that same evening. The last train is at 9.16 PM: you should be able to get it in normal circumstances, but (1) delays happen and (2) you'd get to Versailles at midnight.
If you do choose that option, don't rent a car in Paris. At this late hour, take a taxi from Gare du Nord to Versailles, and rent a car in Versailles when you're ready to leave.

Otherwise, a night in Brussels might make for a more restful trip.

Thank you for the input. A restful trip is important - we will spend the night in Brussels and head to the Normandy area the following day.

I would take the train, recognizing that there will be a fairly lengthy commute in Paris between stations. Trains from the north tend to end in Gare du nord (no surprise) while trains to the west are often from Gare st-lazare, The Metro will make the connection, a taxi is more comfortable. I chose Caen as the Normandy destination and used Trainline to see trips in the 4 to 5 hour range. A wide range of prices, too.

Why Caen? A bigger city, with several car rentals in the vicinity of the train station. It's also interesting, with a war museum on the outskirts and the castle-fort of William the Conqueror in the centre. Definitely take a full-day guided tour of the Normandy beaches. The guide's information and context will more than reward the cost and only an expert could have the same experience freelancing.
A car will help see other areas in this large region.