Unless you install and give all permissions to lot's of not so well known free apps, or surf lot's of sketchy websites, you won't have any problems at all running without monthly updates. Actually doing those things WITH monthly updates would still be ill advised. You need to remember you will always have Play Protect, which is Android's built in antivirus that automatically scans your system and apps. And chrome has built in antivirus when you surf. Android is a very secure os.
Me and my wife have used our android phones just as much without monthly updates as with and never had a single issue with sensible usage. Android has a very strong sandbox design just like your windows phone you used for 2 years with no updates. With all the built in antivirus and safety features Android has the monthly security updates aren't as make or break as folks think. If you read the monthly breakdowns quite often they're just fixing small bugs. They're good to have no doubt

but they're also a good selling point to keep people upgrading