Office 365 is always a temporary rental which they call a "subscription". It comes in monthly and yearly increments. You can "stack" up to 5 x 1 year subscriptions, so it is possible, but unlikely that your 365 subscription lasted for 3 years.
Yes, you can switch to a one-time payment license, like 2010 and earlier used to do. It was (dis)"improved" to allow the license to be installed on only one computer at a time.
You would buy a 2019 license / product key. Uninstall 365, Install and activate 2019
NOTE: when you buy 2019, be sure you are getting a MAC specific one
Uninstall Office 2016 / 365 – 2007
https://support.office.com/en-us/article/uninstall-office-from-a-pc-9dd49b83-264a-477a-8fcc-2fdf5dbf61d8?ui=en-US&rs=en-US&ad=US
Method 1: Uninstall from Control Panel (or Settings dialog)
Method 2: Completely uninstall Office with the uninstall support tool (SetupProd_OffScrub.exe)
Method 3: Remove manually (Registry Hack)
The Method 1, “Normal uninstall”, is designed with the assumption that you will be re-installing Office. It does NOT delete everything. It leaves configuration files like NORMAL.DOTM and other templates etc. It also leaves configuration
Registry entries. Normally these “carry-overs” are safe. But if method 1 didn’t fix your problem, they may be the cause.
Note:Method 1 includes links to Office version specific re-install instructions
So if Method 1 doesn’t fix your problem, try Method 2. Method 2 does a more thorough job that the “normal” uninstall (aka Method 1). It deletes “everything” associated with Office
except your personal (document, spreadsheet etc) files created using Office
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Boot the computer.
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After the restart go to http://office.com/myaccount
to re-install Office 365.
Here is a link to the MS store for one of the 3 offered Office 2019 bundles
https://www.microsoft.com/en-us/p/office-home-student-2019/cfq7ttc0k7c8?activetab=pivot%3aoverviewtab