Do you have Windows 10 Home or Windows 10 Pro? It is my understanding that you can’t prevent Microsoft from upgrading the Home version.
Your understanding is wrong. In some cases it's as simple as configuring a "metered connection" (depends on the Win10 build#). You can install a 3rd-party firewall (and disable the native firewall) and block all the MS update sites (complete URL lists are available online). Or you can roll up your sleeves and hack the registry. Or teach Win10 Home how to do GPOs, and block updates through GPO.
Apologies to the OP if this sounds harsh but cases like this illustrate why your need to keep regular backups of all your personal files increases in direct proportion to how much it would hurt to lose them all.
Back-ups are to PCs what a helmet is to a motorcycle. You might never need it, and riding bare-headed might feel cool, but it's too late to cry about it once your brains are splattered all over the street.