Workflow that changes item permissions based on current Approver?

Hi All,

I am looking to create a workflow with a process like below in SharePoint Online:

1. User A submits new item
2. Item gets submitted to User B
3. While User B is reviewing, User A cannot edit the document
4. User B approves the item
5. User A and User B cannot edit the item anymore

It looks like it will need to edit item-level permissions. As I know, there is a limit of 5000 unique item permissions.

My idea is, in the List itself, User A and User B has New + View permissions. Workflow changes the item-level permissions; but once approved, will inherit from the List again. In this case, it might prevent the 5000 unique item level permissions to be hit.

Any other suggestions as to how to do this in SharePoint Online? Or it is not recommended?

Thank you