Train from Paris CDG Airport to Frankfurt Hbf?

1. No, RER ticket is from RATP and train to Frankfurt is DB or SNCF.

You might find a train from CDG to Strabourg then continue to Frankfurt.

Me, I would spend a night in Paris, then I could confidently buy a discounted nonrefundable ticket to Frankfurt

No need to assume. # 4 through #7 are correct.

I do likewise, except that my hotel in Frankfurt is just opposite from Frankfurt Hbf, and walk across the street to reach the hotel. That RER "B" ride from CDG to Paris Nord costs 17 Euro. I got the ticket from the machine. Signs point the way from Nord to Gare de
l'Est. You won't be the only one going there.

Even though I am quite familiar with the route you're taking, I don't like cutting it close, so as for the time factor, give yourself 1.5 to 2 hours. Say, you get off the plane at 10 AM, between 11:30 and noon, you'll be at Est entering on Rue de Alsace.

My 2 star hotel is en route from Nord to Est, a very nice, recommendable place at a decent price , ie, will return. spent 4 nights there, only saw French guests the entire time.

Say, you get off the plane at 10 AM, between 11:30 and noon, you'll be at Est entering on Rue de Alsace.

That's really optimistic, in my opinion. The walk + wait + RER + walk time from the public area of terminal 2E (so, after luggage claim) to Gare de l'Est is typically 60-75 minutes. Add immigration + luggage claim, and you are really unlikely to make it to Gare de l'Est less than 2 hours from landing. To allow for possible delays, the earliest I would aim for train departure is 3 hours after landing - and you'll see lots of posts here advocating for 4 hours (which is perhaps overly cautious for gare de l'Est, IMO).
Gare de l'Est is not the worst place to hang out, anyway!

The two time variables are the Passport Control which could be less than 10 mins. or up to 40 mins. If you do have bags at baggage claim, that depends on how many planes are landing at the same time thereabouts..

Good that you're in no hurry to get to Frankfurt Hbf.

I've done your route in the opposite direction, ie, landing in FRA from the non-stop overnight flight SFO at FRA, usually arrived just before 10 AM, had the hot lunch at Frankfurt Hbf, then sat waiting for the next ICE Frankfurt Hbf to Paris Est (I used the Eurail Pass). Once there walked from Est (avoiding that staircase upwards with luggage) over to Nord, where my hotel is located across the street from the station. I wasn't in a hurry to get to the final destination, Paris, either, thereby taking the time pressure off you.

I'll add another 40 mins or so. , the total time is 2:40 hours.

True, Gare de l'Est is a very nice place to hang out, better than Nord. I spent time doing just that , more at Est since the hotel was a bit closer to Est than was to Nord.

a little study of the two major routes from Nord to l'Est ahead of time will let you choose depending on your luggage which to take. The shorter one does involve a significant staircase (down to Est, up to Nord) and the slightly longer and slower one doesn't.

Say, you get off the plane at 10 AM, between 11:30 and noon, you'll be at Est entering on Rue de Alsace.

I also agree this is overly optimistic. It could happen once every forty times, but I wouldn't count on being able to do it (and putting unrefundable money on a ticket hoping this will happen).

No body has mentioned that there is no guarantee your plane will land on time.

Indeed, another factor.

Thank you all very much for your feedback! My trip is still a long way off (next fall), so I have a lot to consider in the meantime.

You can look at the train options using the DB Navigator app (German train app) or SNCF (French train app). There are several options departing from CDG, connecting in either Brussels or Strasbourg. Travel time is anywhere from 4.5-6 hours. The fastest trains going from central Paris take over 3.5 hours. So there’s not a lot of time saved by traveling almost an hour into Paris from CDG, if you catch one of the faster trains from CDG.