Non-Upgradeable 2011 IMAC after High Sierra

Hello,

I have a 27-inch Mid 2011 IMAC with MACOS 10.13.6

using Apple Mail & Safari browser.

2.7 GH Intel Core i5

12 GB 1333 MHz DDR3

Obviously I can't upgrade my OS past High Sierra due to architecture

I am very content with my system performance with COX 150 Mb speed

and I'm using less than 50 GB of my 999 GB memory

I do NOT enable Wi-FI and I do NOT allow sharing

I have fairly strong passwords and back up using the cloud and/or thumb drives,

but I use no virus software.

(Every few months I just take 2 hours to erase my drive and then re-download all

mail, data and apps including Pages, Numbers, Brothers printer software, Fidelity ATP, etc

Since APPLE has stopped supporting my OS with security updates, etc THIS MONTH

I was wondering how much risk there is for me??

Do people have suggestions / warnings for me??

(obviously I prefer to just keep going instead of buying a new computer)

thanx....mark