My rule of thumb was always to allow 3 hours between landing and boarding a train. I often ended up with plenty of time to spare, but often enough barely got on the train, so all I can say is play it safe. Then again, I have no problem sitting in a train station after an overnight flight enjoying a coffee and beginning my people-watching adventure.

Not sure what your question about being charged for luggage is. I've never paid a cent for luggage of any size on any train in Europe. If ouigo or anyone else is now charging for bags, it's new to me.

Definitely go carryon. We just completed a 17 day trip with two small children and managed to bring carryon luggage only. Air France is lenient on the carryon luggage parameters. Saved us heaps of time and gave us a head-start from the rest of our flight crowd. We did laundry or just wore the same stuff, even if less than fresh.

The 10:39 should be fine. We did that in December from Toronto. We had to wait a while but were afraid to book any earlier. There was an ok seating area with food. I mostly remember rolling my bag through a puddle of pee from hopefully someone's dog. I used clorex wipes then but scrubbed it good once we got to our apartment. There are some very strange characters in Flanders train station and the Europe one. This guy followed me and kept asking me about a train but had no bags. I told him it was delayed but five minutes later four cops surrounded him and when we got on the train they came aboard and hustled out four guys in first class. I felt good knowing big brother was watching somewhere. They never checked tickets btw. We were asked many times for money. I loved the M&S in Flander's train station. They had small Christmas cakes and plum puddings.

Ouigo trains DO charge for more than one piece per passenger, that's all. It's not very much so I don't know what difference that makes, it's only 5 euro.

There isn't any way I could manage that size for my longer trips to Europe, it doesn't bother me in the slightest to have a 24 inch suitcase on a train, I do it all the time and mange it myself. I have a good suitcase that is very lightweight (it only weighs about 6-7 pounds). But I make sure I have lots of shoe options (not for fashion, but comfort as I walk a lot) and I usually have to pack for a couple weather possibilites, warmer or colder. In any case, I don't get the obsesssion about carryon. The official carryon size is pretty small, it is NOT 22 inches, it's smaller and with the width and depth reqs, only about a 20 inch suitcase really fits those criteria.

I don't care one whit about getting head starts on anyone, since picking up luggage takes bout zero time at CDG, it's always there when I get past passport control, which is the thing that can take time (and just walking out). I don't know what head start it would give you, versus what, I don't know. head start at what?

Oui.SNCF luggage policy:

https://en.oui.sncf/en/help-en/transport-luggage