Miguel, what do do you do in terms of security updates? Perhaps you're still getting them for El Capitan. Apple continues to support older OS's for a while (two years?) but not forever. In terms of its mobile systems, once a new version of iOS is out, you're out of luck with any older editions in terms of support or security patches.
Apple still occasionally issues security updates for El Capitan, which I have downloaded as they become available. Overall, though, I don't have extraordinary security measures. My home wifi network is password protected and private, and I attempt quasi religiously to avoid 'suspect' websites.
The other thing that helps is doing regular maintenance tuneups of one's system and HD. Onyx, a brilliant shareware program, is quite wonderful at this; and DiskWarrior, which has been around for generations, is equally powerful, capable and simple to use.
I also frequently backup (since my writing is my livelihood) - to backup disks - and frequently I backup between my desktop (a Mac Mini + monitor) and my two laptops - which in turn get backed up. That makes a total of almost 6 copies of anything that's important to me (documents, I mean) - because I am slightly paranoid about losing important pieces of writing to computer or software glitches. Basically if something can break down, it will - that's the rule of thumb I live by - so I learned the hard way to back up stuff that matters to me.
But apart from the regular security updates, I don't do much. And my slightly out-of-date Mac OS runs beautifully, powerfully, quickly and generally efficiently.
Oh, yes, the one other thing which is almost mandatory - is to have a good 'computer mechanic'. I have a great auto mechanic for my cars - and a pretty damn good motorcycle mechanic for my scooter - and having a smart independent 'Mac mechanic' is basically a godsend, so to speak. We aren't all lucky enough to live in proximity to our computer mechanics - but if you have a good one close by, count your lucky stars.