CDG to Strasbourg

The time question is an awkward one, because you do not know how long until you get out of the airport.
You plane could be on time or 1-2 hours late.
Getting through immigration (passport control) could be 5 minutes or an hour. Customs takes no time.

I would allow 3-4 hours from the nominal flight arrival, or buy tickets on the day (which costs more), or spend the first night in Paris and travel on to Strasbourg the next morning. There are more trains from Paris Gare de l'Est to Strasbourg than there are from "Aéroport Charles de Gaulle TGV" (the official name of the station at the airport).

Note:
"I understand we have to go through security before we enter the train station" - No you don't. With very few exceptions (Eurostar, Spanish High Speed trains) there is NO check-in or security for trains. Just walk into the station and onto the correct train.
"We aren't planning to check our luggage" - There is no luggage check-in on trains (again, with a very few exceptions), You carry all your luggage yourself, and there is no size or weight limit other than what you are yourselves capable of carrying and putting into the luggage racks.

Yes, Strasbourg is a good place for a few days. It is a much bigger city than Colmar.

If you are on United, you will land at Terminal 1 and need to take the shuttle train to Terminal 2 to get to the TGV station. There is a direct train to Strasbourg at 12:46, which I doubt you could make, but if the stars align properly, you might give it a try. Otherwise there is a TGV connection at 16:56. Otherwise you will need to go into Paris on the RER to Gare du Nord and walk or taxi to Gare de l'Est for trains to Strasbourg.
This is the current schedule, which may change depending on the time of year you plan to go.
http://www.aeroportsdeparis.fr/en/passengers/access/paris-charles-de-gaulle/terminals-map

I can see that you're coming from California. There are many East Coast flights that arrive at 7 or 8 AM, if you can possibly arrange this. Do you already know how you feel after the long flight? Maybe you want to try a different schedule, or stay in Paris instead for the 3 days?

Note that Sandy is asking about the Strasboug - Colmar comparison since they are already scheduled to stay 3 nights in Colmar with the tour; including a daytrip to Kaysersberg.

If you like vehicles, my parents recommend the auto and train museums in Mulhouse, 1 hour by train from Strasbourg (runs through Colmar on the way).

Have you looked at United providing a code-shared flight connection to Strasbourg (or Mulhouse/Basel)? That way, if there are flight delays, they're still responsible to get you to the end destination. But if you bought a train ticket in advance and missed the train, you'd be out that money and end up paying more for a new ticket.

I don't think there's any real similarity between Strasbourg and Colmar, except possibly for regional wines and food, but that's a plus. The cathedral is wonderful, the Musée Alsacien was very interesting. I joined a "free" walking tour that began in front of the cathedral, around 2 p.m. I think, that was very good. Have dinners in Petit France, lovely place to walk around too. Take a canal cruise. There's certainly enough to fill 2 full days. My favorite drink was kir (local white wine and cassis). Even better was kir royale aka kir crémant with bubbly white wine.

Whether you book the train from CDG to Strasbourg in advance depends on whether or not you are gamblers. If all goes well, you save money. If there's a snag, you are out of pocket for the price of the tickets.

I would consider getting to Reims a day before the tour begins. Two things I enjoyed very much were a day trip to Epernay, including the Mercier tour (rather Disney-like, interesting and fun) and visiting the Musée de la Reddition. I took two champagne tours in Reims. They were quite different from each other and from Mercier. All three were enjoyable. You will want to dress warmly for a cave tour.

I wish the OP had added "[Colmar] ... which is on our Eastern France tour", because I had to do several clicks to find that out.

I agree with the previous poster that Colmar is not really the same as Strasbourg. That is, three nights in Strasbourg are not wasted on a trip that includes Colmar. After reading through the itinerary for the Eastern France tour, it seems to me that this tour does not really cover Alsace north of Colmar. That leaves a lot to see, if you can arrange transportation. (We had a car for Alsace.)

Also, it appears that an RS favorite, Besancon is not on the formal tour. I believe that Metz and Nancy (haven't been to either) might be good day trips from Strasbourg. I also liked Luxembourg City and Freiburg. Indeed, if you are "active" 70's, the Black Forest might be really nice in August. But note that you have to take the European vacation season into account in your planning.

Regarding your CDG arrival, I don't want to panic you, but the day we arrived to take the TGV to Strasbourg, there was an abandoned-bag or something in the CDG train station, and we were kept outside for almost an hour. Luckily, we did not miss our train. I don't know what the accomodation would have been if we had missed it. We certainly did not want to go into Paris and transfer our bags to the regional rail station.