I have heard good things about openoffice, but I still have Office (old version, pay one price - not a subscription to 365).

My kids have taught me, though, that if I ever have a problem there is probably a You Tube video for it. Back when Microsoft was installing Windows 10 without permission, I searched Google and You Tube to find a program that protected my computers from the W10 download. It would only download if I gave express permission. I'll bet you'll find something about disabling the Office 365 popups by googling and You Tube search.

I have not seen those popups. I wonder if my AdBlock is blocking them? It's not blocking other installed programs from reminding me that I can buy premium versions, etc., though.