A competency test is just the start of the end of a democracy tbh,

Not inherently

someone can influence political decisions by making a specific political idea "incompetent"

Correct, which is the biggest issue and why it's very dangerous to implement in practice.

That being said, there is nothing inherently wrong with competency tests. We have competency tests for driving, practicing medicine, law and so on. Why? Because we don't want incompetent individuals with the power to cause a lot of damage.

Through voting, millions can be stripped off health coverage. Wars can be started. The power to vote can create or destroy entire civilizations.

A basic competency test for:

• ensuring that the voter understands the structure of governance
• can identify logical fallacies
• can identify cognitive biases

would help reduce the probability of disaster imo.

The question of "the test setter can act in bad faith" can be remedied through independent sortition based committees requiring a supermajority approval for the test. I mean I can't see how designing such a structure in theory could be that difficult.