Advice sought to build document review workflow in Sharepoint

I think Microsoft has launched "Approvals" for this kinds of flows, which is quite simple to use but unfortunately does not work with "Guest users" (at least it doesn't work in our case).

In our organization, I have created a set of flows to send automatic reminders when a due date of a review process has arrived. People ignored it many times, so I have added another step that requests them to modify a cell when they complete the task in the reminder. People ignored that too. So I make it as a loop that when the recipient does not modify the cell, it sends a reminder mail every day. But then, it keeps sending emails even though somebody is away for a sick leave... It went like this.

It is so surprising that there is no simple solution for such a simple request.