Welcome to the forum. The fact that it comes on makes me think it’s not brushes. Not coming on or running at a reduced speed is more indicative of brush problems. It sounds more like it comes on just long enough for something like a capacitor in a circuit board to heat up. If you are knowledgeable in that area you could try bypassing the soft start and speed control and see if that solves the problem.
You may be right that it is the speed controller.
This model is known from customer complaints to have
1. Non-dust proof switch that many have reported have reported malfunctioned when dust got in
2. Weak electronics - speed controller failure
But what I proposed in my previous post is UNKNOWN to most people.
It is the shorting of the armature when the gaps are filled with carbon dust.
Unless the armature gaps are already cleaned up first, there is NO USE trying to bypass the speed controller YET.
As I said, I have repaired my drill and vacuum cleaner by just cleaning the gaps in the armature - the speed controllers had no fault.
In the case of my Hitachi M12V, just cleaning the armature gaps allowed me to resurrect it and run for a few months after that I found that I really had to replace the speed controller. Unfortunately the speed controller cost more than buying a new cheaper brand router.
So the sequence of repair important.