I don't understand why Minion necro is allowed to exist, even as a meme build, while turret engi must be kept completely unplayable.
Both builds rely on AI to deal the bulk of their damage. However, minions have two key advantages over turrets.
Turrets are completely stationary and, unlike minions, you can barely empower them with traits. Ideally, this means that they should be Stronger than minions in a stationary battle, but it's the opposite. An engi running 4 turrets and supply crate loses in a 1v1 to a minion necro. It makes no sense that turrets should have more trade-offs than minions, yet still be weaker in their most ideal scenario.
So I propose the following changes. If the issue with turrets is that we don't want to promote heavy bunker-down gameplay, then I suggest we give engi turrets the ability to fly. Rework them into drones that follow the engineer around the same way that minions follow necro around. Let the engi use their abilities on command, and rework toolbelts into detonate skills.
Alternateively, get rid of them entirely and replace them with some other utility. If you're going to give up on balancing them, then replace them with something you can balance.