Quiet, reasonable place to acclimate

From CDG you can get a train direct to Lyon in 2 hours or or smaller town Nimes in 3 hours.

I think the Loire valley would be wonderful. Pick any town along the train route Amboise etc

To get away from popular tourist sites, you'll really have to get off the beaten path. Since it's your first trip to Europe, you might want to compromise a bit. I agree that the Loire Valley would be a good place to chill.

Since Venice is your goal, I'd recommend Dijon or Beaune. It's on the way, small towns, relaxed vibe although touristy still. But two nights, not three.

Now, if you want somewhere off the beaten path, I opened my "random town generator" and came up with Arc-et-Senans. Southeast of Dijon, on the rail line towards Switzerland and ultimately Italy, this town boasts a stunning masterpiece of 18th c architecture (Google it) and offers a very French village vibe. Tiny village though. I had some excellent food at the Relais d'Arc et Senans, if the rooms are as good as the food they must be a treat.
Besançon is daytrippable.

Again, I'd recommend no more than two nights, such a tiny place. And for a first trip to Europe that's only nine days, it would be a really odd choice. But you asked about off the beaten path 😉.

Since this is your first trip to Europe let me point out a few pitfalls you may not be aware of. First, planes can be delayed and the time thru passport control can be unpredictable. So problem #1 is you can't buy an advance purchase train ticket on the faster lines to a town a few hours out of Paris without building in a wait of several hours at the airport or taking the risk of having it expire worthless and having to buy a much more expensive walk-up ticket. You can sidestep this a bit by picking a town within an hour or so by a slower train such as Rouen (beautiful town, BTW) which don't cost more for same-day tickets. Which brings up problem #2. Train service from CDG to the rest of France is relatively infrequent. Usually you need to go into Paris to one of the main train stations. It isn't hard to do this by RER but as a first-time traveller it might be easier just to take a taxi.

My advice is to spend the first nite in Paris. Take a taxi to your hotel. If your flight is late, if traffic is bad, it doesn't matter. The next morning take a train using your cheap advance-purchase tickets to the town for 3 days & 2 nites. Alternatively book flights so that you arrive somewhere like Lyon instead of CDG

I suggest Colmar! Charming, easy town to visit from Paris for a couple of days. https://www.ricksteves.com/europe/france/colmar

Not Colmar you would be arriving when all the tourists are crowding the town

Not sure why you don’t want to stay in Paris if this is your first trip to Europe,
That being said. Nimes is not bad but it is not exactly a small town.

Personally I agree with the Loire valley suggestion. I stayed at a nice little chateau outside Tours it was reasonably priced and had a good restaurant in the main Chateau itself and some nice grounds to walk around,
The downside of f this area (and Nimes or any small town).) is that you have fewer transit options. So if you want to go someplace you will need to rent a car or take a tour.

What time is your flight scheduled to land in CDG? Unless it's early in the morning, I would spend the first night in Paris. Then, I would take the train to Avignon, or maybe Lyon; stop in Torino; and then make my way to Venice.

Do you have 9 days to make your way to Venice, or is 9 days all the time you have in Europe?

I second (or fourth) the Loire Valley as a place within a few hours of Paris (albeit in the opposite direction from Venice). Base yourself in Blois or Amboise, and see the Chateaux of Chambord and Chenonceaux.

For a place enroute to Venice, perhaps the Burgundy region? I've never been but always wanted to.

I’d also warn you how much time it takes (and sometimes taxi money too) to change accommodations. Especially in a city you don’t know your way around, and on a first trip to Europe. This news board, and our host’s books specialize in helping first-timers, but you need to read (for example, free travel hints in top left blue menu) and consider the advice.

Unless you are flying home from Paris, you should stay two to three nights there upon arrival in Europe. Your interest in a small town is reasonable, but it’s not a good plan for your first arrival in Europe. Do you wish to skip Paris entirely for some reason? Are you worried about safety?