No reason to give up Mojave hust because Apple stops patching it.
Number 1: The odds you'll be victimized by any of the few mac exploits is extremely unlikely.
Number 2: If you are concerned ... there are a number of third party mac anti spyware products that can give you peace of mind
Staying as close as possible to the most recent updates on your devices/computers is the best protection. In addition to the ever changing malware and spyware variants (that still can easily fool endpoint products on up to commercial IDP/IDS/NGFW/UTM systems by changing a couple of bytes), there are plenty of new critical/high vulnerabilities that crop up across all operating system platforms and the 3rd party applications that run on them with regularity.
Beyond the devices themselves, I use an enterprise firewall platform at home, two raspberry Pi's (to sinkhole nefarious dns lookups), Wireguard VPN and other fun things. Even with all of that in place, I do not access the internet with any systems that can no longer can obtain updates.
I have a 24" white iMac and two Mini's from years ago that are running in isolated enclaves on my LAN at home with no default gateway set. This allows them to connect to other devices on the same LAN but nowhere beyond.
BSD UNIX and Debian Linux are among the most secure... macOS, not as much.