2011 (late) 27" iMac

These days, Apple ties Safari updates to macOS updates – so you're stuck with the version of Safari that shipped with High Sierra. For some sites, especially financial Web sites, that version may be considered a bit out of date.

You may want to consider installing and using an alternate browser, such as Firefox ESR. Mozilla no longer offers support for High Sierra on their main channel, but their support announcement says that Firefox 115 ESR runs on High Sierra and will get "critical security updates" until September 2024.

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

A 2011 iMac is considered obsolete. It can't run any thing higher than High Sierra 10.13.6 which is already showing its age. It's time to consider replacing it.

To answer your question regarding security, yes you can by using a variety of complex passwords for your internet sites and Wi-Fi, etc.