> Also, for example TGV requires you to be at the check in 20 minutes before.

Where?

For TGV Inoui (the main brand) you have to be in the train 2 minutes before (written on any ticket, also here: https://www.sncf.com/fr/offres-voyageurs/voyager-en-train/tg...), and that's all. People flow through the gate up to the last minute. It has been like this for at least a decade.

For TGV Ouigo (low-cost brand), it seems to be 30 minutes according to https://www.ouigo.com/faq?question=ai-je-pense-a-tous-avant-...

I never understood this constraint with Ouigo though; it's obviously possible to skip it as shown by Inoui trains... and both are operated by the same company from the same train stations.