Usually there's no advantage in buying tickets in advance for short distances, in terms of money savings, especially since you want flexibility.
The way I decide (when planning my trips), is by checking the routes I'd be taking, & choosing tomorrow's date, then comparing the prices with a date a couple of months from today.
When I checked your routes, the only one that showed a higher price (therefore may want to book in advance) is the
Tours>> Chartres, but only if you take the TGV via Paris. You may want to choose the TER train (or Autocar) that departs from Tours Centre station.
Have a wonderful trip!
EDIT: For your Paris>> Rouen trip, you'll take an Intercity train that offers €10 tickets if booked in advance, vs. €24,10 (normal price) if purchased the day of travel. This is not a big difference in price, & worth it, if you want flexibility with your itinerary.