Train ticket from Paris to Bayeux

You can wait and buy the ticket on your way to the station or at the station. Trains run about once an hour. It is possible that the trains might sell out so that is the chance that you take by waiting. How likely is that? I have no idea. I was just helping someone buy a ticket to Bayeux for the 24th and there are no seats available until 8:59 PM. Personally, I would buy flexible (i.e. modifiable) tickets now and pick a train that leaves four hours after my ETA at CDG but that is because I don't like having to find a ticket machine or wait at a window to buy a ticket. I get the ticket that is modifiable until the train leaves which costs more than other types of tickets but I totally hate the idea of not having an assured way to get to my destination after flying for hours.

Indeed as suggested buy a flexible ticket with a generous margin.
You do not even have to exchange the ticket to take a later or earlier train. You just won't have a guaranteed seat in that case. But since reservations are only offered to/from Paris on that train you can just look for an unoccupied seat as the train is getting ready to leave.

I was just helping someone buy a ticket to Bayeux for the 24th, and
there are no seats available until 8:59 PM.

The 24th is a unique situation, because of Storm Austen which is currently hitting Normandy all trains in the region have been cancelled today the 23rd. Consequently, all passengers who were supposed to travel by train today have rescheduled their trips for tomorrow.

This situation will not be repeated on the 29 (according to the weather forecast).

I have taken that train many times and I have never heard of the trains to Normandy selling out. But I used to buy one a couple of hours after I estimated I would get there. If I was early, I would sit in a café across the street and settle into France. There are also several dining and shopping options in the station itself. These days I tend to spend one night in Paris before heading out to Normandy.

Thank you all for your suggestions, really appreciated!

I travel in the summer in France, some in June and mostly in July.

If you want to get the ticket Paris to Bayeux r/t last minute, say a day or two prior to your desired day, even on week days , let alone on the week-end, you can expect one or both rides will be sold out...almost a given. i saw that numerous times since 2023, also signs posted to that effect at Gare St Lazare.

For your trip on 29 Oct, however, you are ok.