@jaysf possible that his is what you need to do. My concern with all of this is that the symptoms you describe can also be caused by water damage. External cleaning and letting it dry etc. may not be enough. I would suggest you treat it like any other water damaged phone.
For now you want to stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse.This may further damage your phone.
The next thing to do is to clean it. Disassemble your phone using these guides [[Topic:Samsung Galaxy S9]] . You must remove all EMI shields. If you do not remove those, you might as well not do anything. After you removed those, clean the whole board, connectors and all with +90% isopropyl alcohol. Follow [guide|5825|this guide] , and even so it was written for a 3G all the points are still pertinent to your phone. While you clean your board, check for any obvious damage, like burned or missing components etc. When it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Best thing to do would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your phone and reevaluate. Until it is cleaned, everything will only be a guess. It is possible that you have a bad LCD and replacing the display assembly (or the flex if you have the ACF Bonding equipment and machinery which will set you back a couple of thousands) may fix it.