Moblin and gang go away

We're playing homebrew version of the dragon of icespire peak. Beefed it up because everybody was tired of playing level one. And skiped ahead to level four. Doing so we beefed up the encounters as well, such as Cryovain. Added support guard drakes. But for a level 4 party of 6, they nearly killed the dragon in 5 turns.

I believe that some their abilities they may not understand, And so I need it to look at the character sheets and find out what the monks can actually do, but they told me they could disengage every turn after an attack without triggering an opportunity of attack because of the mobility feat. And as far as 5E guidelines seem, unless they're talking about something else That's not how it works. But it was a totally unfair fight, I didn't want them to wail on the dragon, but they reduced it's size then grappled it, leaving little for him to do.

I know better for next time, but now I feel like my players feel they can plow thru anything. They aren't scared of a dragon, they aren't scared of the bandit gang in town. They are about to head face first into a bandit hideout without a plan, how do I give them a healthy fear? I sent them to a shop to get gear and All of them refused because there wasn't any good gear. They have plenty of money I've made sure of this. But now that they have so much money they refuse to spend it. They need a reality check, that they aren't gods