iMac too old to upgrade to Mojave

I don't think you mean Mojave, which your computer should run. Are you referring to Ventura, which it will not? The latest OS your iMac is capable of running is Monterey, which will be supported with updates for some years yet and is no cause for concern to a user.

If the Mac does what you want ti to do with the system it has not there's no reason to not continue using it as it is. Follow the recommendations in these documents and you'll be fine:

Effective defenses against malware and other threats - Apple Community

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

As you've been told your Mac can run Monterey and that will be given any necessary security updates for at least a couple more years. You're in good shape.