Just use the known quantum hardened crypto algorithms.
You’re being a bit too paranoid. If you have such important data to the point where MS bothers using quantum computers to decrypt it, then you shouldn’t use any cloud service in the first place. (Maybe not even touch the internet).
Who says I’m keeping anything in MS or having important data? 
Privacy itself is important!
From “paranoid” perspective, storage is cheap (at least for Microsoft, like Jon pointed out with competition’s pricing), so they could easily keep everything until quantum computing is fast and cheap enough to decrypt (and most users don’t encrypt their files). It’s a matter of individual risk/benefit ratio.
Besides, it’s not a matter of being paranoid, but knowing that those companies often run on people’s data in ways and with consequences that users don’t know or understand (until it’s too late).
I wouldn’t have wrote here if it wasn’t for a contrast between Jon’s suspiciousness about third party OneDrive clients (not saying that being careful is bad) and no such doubt about keeping his data in MS.
Let’s end with a classic quote from the times when paying for services with your data was in its infancy:
Hi Jon,
As the developer of the OneDrive Client for Linux I can advise you that you never ever provide the client with your MS username and password.
The way the client works, you authorise the client using a token that the authentication process, using the Microsoft Authentication API provides - then that token allows the application to access your data.
No credentials are sent or used or stored or even provided to any system anywhere. Feel free to review the opensource code on GitHub as well.
If you are still uncomfortable, happy to setup a conference call MS Teams meeting and show you what is going on.
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