I am a French national with a valid passport and a 60 days tourist visa wich I got extended for 30 more days at the Chiang Mai immigration office. Can I get 30 more days by going out and in in Mae Sai? Thank you for answering, especially if you did it recently in person.

You can get a 30 day visa exempt entry after having a tourist visa without a problem. You will get a 30 day entry since you are from France (G7 rule).

You can now extend that 30 days for another 30 days at immigration.