Taking out the motherboard and removing the shields to soak in ISO (95% or greater) for a bit and gently scrubbing with a toothbrush is definitely helpful. However, you would want to decontaminate it by cleaning in an ultrasonic cleaner and rinsing well in the ISO.
Then, after letting it air dry for a few hours (to overnight) you would want to begin inspecting the board (preferably under a scope) to look for obvious signs of burnt (melted) under (Over)fill, signs of shorting (arching) on or near components...Probe with multimeter for shorts or possible component damage...
Sometimes, with water damaged phones, you can only hope to restore to do Data recovery due to the water getting under IC's and causing further damage down the road.
But taking to an experienced tech (Micro soldering) for inspection and diagnosing would be the best thing if you don't have an ultrasonic cleaner, microscope or micro soldering equipment or tools.
You have done more than most by simply cleaning with alcohol and not putting in a bag of rice! :) But still falling short of being able to properly decontaminate and clean the Logic board to begin the search for the damage.
If you are handy with this type of repair and have a few basic tools of the trade, you may get lucky and repair a blown filter, short, or even removing and replacing a faulty IC... but in my experience, even an experienced tech can fail to repair completely do to not being able to foresee future issues with corrosion under components that have no current issues.
Best of luck to you and I hope the repair goes good for you. If you can't find a local Micro Soldering repair shop, there are several here in the Ifixit Pros area that you could do a mail in service.