Internet protection for my 2017 iMac 27”.

Short answer: no.

Longer answer: There is no 100% secure OS. Security is baked into MacOS at its core. You're more likely to be a victim of a phishing attack (someone tricks you into giving up your password) than you are to have a malicious package find its way into MacOS. 3rd-Party anti-virus programs usually cause more harm than good.

Third party Anti-virus typically cause nothing but added issues with the macOS, so not recommended.

ref:

macOS - Security - Apple https://www.apple.com/macos/security/

Apple Platform Security - Apple https://support.apple.com/guide/security/welcome/web

Here is a common sense guide:

Effective defenses against malware and other threats

https://discussions.apple.com/docs/DOC-8841