Sharepoint archive with limited rights for deletion

We have following use case:

- there is folder structure: year/month/invoice, eg.: 2021/01/ATB00123

- user wants to have files on a sharepoint library and to work with them, also sync for the current month;

- admin wants to lock all previous month folders to prevent from deletion;

- prefarably locking must be done manually; 

what is the best aproach to implement this into sharepoint?