Attacking units surround attacked building properly? (New solution, or fix to Nav Mesh?)

Hi guys, sorry for my stupid question, but I couldn't go forward in this problem since morning. I've been using Unity's Nav Mesh Obstacle and Nav Mesh Agent, but there were problems, like running in place, facing against each other, and running in place, or just teleporting. I am working on a MOBA, and I thought that I dont need a full pathffinding system for the creep's AI, so I decided to make my own. But I can't really think about a good, and performance friendly algorythm.

Could you guys give some tips, or some plugins? I've been searching and testing the whole day.

My game currently is this: Creeps go from Team A's castle to Team B's castle on a straight line. If they find an enemy unit in AggroRange (enemy creer or building) they start to move forward the enemy unit, and when they get in range they start to attack it. It was fine with Unity's Nav Mesh, but it wasn't even close to perfect. So I want to find another way to make the creeps move smart. The only obstacles in the scene are there units, which are destroyable, and for example if they want to reach the second tower, they must destroy the first one. (because the tower agrroes the creeps)

So what I need is simple: Only 1 unit can be in one place (so the units got colliders), and they can effectively attack enemy units. (for example when 30 creeps can constantly attack a turret, and currently 29 attack it but a new minion comes, they reassign, and they let the 30th minion in, so they can attack the tower at max effectivity)

Sorry for the long post, but I am so lost in this topic. I can't write a proper one at my own, and all of the currently available ones at the store give more they I need.

Thank you for your further help. The best solution would be fixing Nav Mesh, because it was really easy to use.