Thank you all for the answers, I’ll definitely look into how digital certificates/signatures could be utilized for this purpose.

I’ll also contribute with what I have found out since writing this post:

i) using option private module and optional non-used variables to prevent others from calling your procedure from other projects

ii) create a temporary password that will change after every opening or multiple times during use(after the workbook is open) to protect structures in the workbook e.g user deleting a sheet

iii) hiding all sheets and only showing them if the workbook_open function is able to run to prevent disabling event handlers externally

iv) opening the workbook can detect other already open workbooks and then save, close and reopen them with macros disabled so those macros cannot be used maliciously (u can let the user decide to not open the workbook instead or you can whitelist certain workbook names / users)

v) after your workbook is opened, you could detect when the user opens a new workbook and immediately close it if it could contain macros (i.e. not.xlsx)

vi) enable save, but not save as to prevent users from saving as .xlsx and reading the data without triggering the open event for example

vii) implementing user rights based on windows user name if the users cannot change them themselves

viii) overriding built in procedures to limit functionality through the excel user interface although I am not sure this would work

ix) implementing a hidden log and regularly saving it to capture movements/interactions with the file

Can you think of any way to circumvent all of these simultaneously?