After having downloaded from the website office.com/setup the Office 2019 pro plus version trying to activate it (owning a valid product key) by opening a new word or excel session I do not get prompted the activation steps! Instead a standard word or excel session is opened, under Office 365! I then attempted to uninstall the Office 365 version ... but mysteriously my windows 10 does not find any! I created then a second Microsoft Account e re-attemped the activation of the Office 2019 ... but AGAIN by opening any Office app (an excel sheet in the second attempt) a peacefull new exel sheet under Office 365 was opened denying me again the possibility of activating the Office 2019! I'm going mad! Any help?

Did you follow Microsoft's instruction for uninstalling Office 365? See Uninstall Office from a PC

Also try switching from MS account to Local account? See https://support.microsoft.com/en-gb/help/4027068/windows-10-switch-your-device-to-a-local-account

You can always switch back if it doesn't work. See Switch from a local account to a Microsoft account

I am no expert in Office 365 but these seem worth a try.