I should mention that if your PC doesn’t support Windows 11, then it probably? lacks firmware updates, and it has no basic hardware security features. You should ideally upgrade your setup, but the previous recommendations are only harm reduction.
ChromeOS Flex lacks full verified boot, but that’s basically every PC except chromebooks and some macs. You can use it with secure boot as a better than nothing measure. I still don’t see why anyone use it, unless if they don’t like to use a phone for internet browsing, but I’d personally air gap this PC and use it for storage, or maybe connect it to the internet if it has no personal files or photos and never use it to sign in to accounts (also cover the camera and microphone)