I mean, when you level up, low tier enemies tend to vanish from the game leaving you with only tougher enemies to fight, this is logical but it diminishes the variety of enemies you face when playing as a high level character. Is there a MOD that implements support for the enemies that vanish as you level up?

Yes. Several.

OOO has enemies that are leveld based on distance from roads and towns etc. So near towns and roads you get low enemies, and far away they are higher. It does an awful lot more than that but that's the main of it. There is also FCOM which is several overhauls combined, but it's not for beginners.

TIE makes everything unlevelled, so everything is completely random and you can encounter all enemies everywhere.