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)

让议员真正私密

Falco (Falco) 2020 年12 月 3 日 15:12 2

话题		回复	浏览量
How to make it so that admins are not able to view private messages of their users? Support	10	1987	2019 年5 月 9 日
How alert if admin/mods read users PM? Support	9	1307	2022 年2 月 14 日
Are PMs truly private and not crawlable? Community Building	4	1311	2020 年8 月 16 日
Can I view private messages as an Admin? Support	13	5261	2020 年9 月 14 日
Can true private messages be implemented? Feature	28	5770	2020 年3 月 9 日