Office 2019, version 16.78.3 compatibility with OS 26.6.2

Your Mac mini (M1, 2020) can run macOS Tahoe 26.6.2, and Office 2019 supports Apple silicon. (Microsoft Support)

For your standalone Office 2019 licence, version 16.78 is the last Microsoft build that supports Office 2019 licence types. 

Your version 16.78.3 is therefore within that final supported build series. (Microsoft Support)

The difficulty is that Office 2019 for Mac reached end of support on October 10, 2023. 

Microsoft currently supports macOS Tahoe with newer Office builds, beginning with version 16.101, but does not state that Office 2019 version 16.78.3 is supported on macOS Tahoe 26. (Microsoft Support)

This does not mean that Office 2019 16.78.3 will necessarily stop working after a macOS upgrade. 

It means that Microsoft provides no current compatibility assurance for that particular combination.

Bottom line: If continuing to use your standalone Office 2019 licence is important, there is no Microsoft-supported basis for saying that upgrading to macOS 26.6.2 will preserve full Office 2019 functionality. Your current macOS 15.7.7 + Office 2019 16.78.3 configuration avoids that unresolved compatibility question.

I would make a current Time Machine backup before upgrading, regardless of which option you choose.

Microsoft — End of support for Office 2019 for Mac

Microsoft — macOS requirements for Office for Mac

An alternative to MS Office could be LibreOffice which works much like MS Office 

Just be sure to download the Apple Silicon version 

The problem may be that Microsoft usually deactivates their activation servers for Office versions they have declared at end of life. Even though the computer OS may be compatible and you have a proper key, you may not be able activate.

It's been that way quite a while. I encountered that in 2018 when trying to install a legal copy of Office 2008 on a new Intel iMac running 10.13 High Sierra. No joy. I had to buy a newer Office version.

One consideration: Before resorting to giving MS more money, I tested both LibreOffice and Apple Pages and found both changed some formatting when opening MS Word files. Mostly margin issues but it also affected placed images and graphics. Had I but a few dozen Word files, that would have been a non-problem. Unfortunately, I had thousands.

I have Office 2019 running on Tahoe. I don’t use it much, but all the apps open when I need them.

Madinmumbai wrote:

I will give it a try and get the OS upgrade. If it doesn't work, I might have to switch to an alternative. Unfortunately, Pages is not very popular within the environment I work in, else it would be a nobrainer.

You can export from Pages into Word format when needed. That's what I've been doing for a long time.

Madinmumbai wrote:

After the upgrade MS Office is working just fine.

Thank you for the updated information