Tracking whether managers actually complete their review cycles on time

We had exactly this problem. The issue isn’t really sending reminders but knowing who actually needs one without HR constantly checking spreadsheets.

You can build something in M365 with Forms/SharePoint + Power Automate. Store review status in a SharePoint list, trigger reminders based on due dates, and maybe throw a Power BI report on top for completion rates. It works, but someone still has to build and maintain the whole workflow. That might be a bit of an overengineered solution

For us, the bigger improvement was using a performance review tool that already lives in the Microsoft ecosystem. Teamflect is one option since it runs inside Teams/Outlook and actually keeps track of all that data. That said I'm sure there are others in the app store. You don't have to build every piece of automation yourself. 

I’d definitely automate this either way. HR manually checking Excel every quarter and chasing individual managers is one of those processes that becomes ridiculous very quickly once the company grows.