In the days when I ran a 5e game I pretty much had to rewrite the monsters on the fly, if I didn't even the legendary foes were pretty much speed bumps. Pretty much every monster had their damage dice bumped up one die or sometimes doubled. Most had their AC bumped up 2-4 points and were usually given max HP. Often I'd give them special abilities on the fly, additional save or get hosed effects. I'd say "this Ogre is tainted by Juiblex and has some rot effect when he hits", etc.
I can't say I spent a lot of time outside the sessions doing this, as I said it was pretty much on the fly. RP wise I played them as they were supposed to be played IMO.
