Which MacOS best for security?

Hi all

I am only interested on upgrading the OS to have my iMac 2017 5k up to date when it comes to security. I do not need the newer features that come with the newer OS.

With that in mind, am currently running 13.6.9 which is the latest native version for my iMac and I understand is good and should be receiving security patches for 1 more year? is this correct? When this support is dropped, would I be best going to the latest MacOS 14 would be at that time or the latest MacOS 15? I am a bit lost on this.

Thanks!