I personally wouldn't be overly concerned, the OS will continue to function; no more support from MS just means, in effect, no more patches or updates. This will leave your machine at no greater risk than it is at the moment. I have never used MS firewall (rubbish), always relying, as I do, on a third party virus/firewall suite. (I use AVG) I'm not sure if your virus program has a firewall built in but any of the better virus protection programs, such as AVG, my favourite, will keep you safe. You should be able to keep using your current setup for as long as it continues to meet your computing needs.
OnePlus 5 review