Giant centipedes and other vermin like rats and bats, and monsters like rot grub or green slime are better handled as traps. They don't need to be in the monster manual.
Talking about giant beetles and how I used them: pack animals for the underworld, war elephants for goblins and the like, or car sized monsters in dungeon corridors
On the things I do for prep: random tables, monster lairs, jotting down some shallow ideas, run with it in such a way that I don't need to improvise in depth but that shallow encounters point to other shallow encounters, take note of what happe
Things to consider when writing RPG text: keeping it short, keeping stats short, keeping it all in one place, don't require leafing around, have art that referees can show to player, have maps that referees can annotate
The mini settings I like to generate where the environment or the map generates inspiration for conflict: the river valleys generate political entities, the swamps and mountain peaks religious entities, add secret societies, a long war somewher