I have spent years running games of 6-12 players for a multitude of game systems. First off, players doing rational, if unexpected things, is awesome. And it happens more often in large groups, ime. Their ideas shouldn't always work as expected, but if there's some sense to it, it should do something, even if only for a round or two.
Second, luck happens to players and GMs. Sometimes a round or two is all they need. And if it shatters their folding boat, well, that's a worthy use of treasure.
Third...well, loners are gonna be loners if you are wedded to a concept. But loner and "by themself" aren't the same. Maybe the dragon keeps piercers as pets. Or gelatinous cubes.
One thing I avoid is using the "mega monster" approach except for rare occasions. It is very hard to tune power levels with any predictability. And as you have found, a mob of twenty is a pain. So go with 3-ish monsters: a boss and two minions. The boss is a role, not a power level. Maybe its a kobold that has two trained bears. Or an tiefling warlock and a pair of ettins. Or it is a dragon with a pair of pet dire snakes. Maybe a wizard, her warrior henchman and a pack of hunting worgs.
The point is it changes the action economy to something a bit easier to manage, creates opportunities for the bbegs to assist each other, and is easier for you to tweak to provide a wider array of challenges for the different members of your party without having a "super" monster.
Plus, if one of them escapes, they have the chance to become a recurring problem, or at the very least, make it a multi-scene fight.