I'll probably regret saying, but I've been running High Sierra forever, and plan to keep doing so. Every time I've upgraded, I've run into problems and either lost the software I bought, or upgraded to the new versions which took away features I needed and added ones I didn't (looking at you, Scrivener).
I run Little Snitch and generally feel pretty secure with that. Caught an annoying notification attack just the other day. Not sure why upgrading to a new system is needed, but happy to have my ignorance un-ignored.