Office 365 is either a monthly or annual subscription, either way, it's a subscription.
But you can buy an office 2019 (which just came out) as a one off purchase and then its valid for as long as you have it, its more expensive for the same software (the difference in software between Office 365 and 2019 is your ownership and cloud functionality, but thats it otherwise) but that makes sense, as its less of a serviceable return for Microsoft.
I have access to discount for Microsoft software (which makes sense, I work for them) - however I cannot resell it. Thats the only thing I can't do with the keys. But I do know keys are floating around on eBay which people resell. Thats probably the absolute cheapest way and would likely be fine for years. If I were in your shoes, thats where I would go.