Is there a way to make MP’s actually really private ? Where an admin can not access other members MP’s unless there’re invited with a flagging or a tag ?

Yes, check Discourse Encrypt (for Private Messages)

Rendre les députés réellement privés

Falco (Falco) Décembre 3, 2020, 3:12 2

Sujet		Réponses	Vues
How to make it so that admins are not able to view private messages of their users? Support	12	1945	Septembre 8, 2022
How alert if admin/mods read users PM? Support	10	1257	Mars 16, 2022
Are PMs truly private and not crawlable? Community Building	4	1290	Août 16, 2020
Can I view private messages as an Admin? Support	13	5198	Septembre 14, 2020
Can true private messages be implemented? Feature	29	5694	Mars 9, 2020