The apple store have not got the know how to change the top flex assembly. They will replace the screen with the assembly already installed and hook it up to their pairing machine to enable face ID. I would try cleaning and repositioning the face ID components first.
I have never run into this problem after an audio IC repair. It is most likely around the connector area there are a few filters on both sides of the board that might not be making good contact. Also check the two power lines for touch when it is not working and also that no pins are pushed into the plastic of the connector.. The ideal tool for this diagnosis would be the iBridge. You didn’t mention where you put pressure on the board for touch to return, if it is on the CPU than you will need to re-ball it.
Firstly don’t completely rely on Tristar testers. You will need to rule out the dock assembly. What does the battery info show in 3uTools? For the XR I have only had to replace the Tristar once it is usually Tigris and or the charging coils I now replace them all. The 8 plus I would replace Tigris and maybe Q3350.
A water damaged iPhone 6 will eventually fail as the corrosion takes hold. In the meantime I would replace the charging dock connector. They are only a couple of dollars and will probably bring your sound back. While you have the phone open I would lightly scrub the logic board with 99% Isopropyl alcohol.
A new battery might fix it, if not it will be your charging dock assembly. If you are going to replace the dock assembly make sure it is a genuine one pulled from another board as some aftermarket ones have faulty or no sensors which makes the phone restart every three or so minutes.