The only step left that’s realistic for a novice, aside from having someone with board level repair experience look at the phone, is to try replacing the battery. Batteries are very vulnerable to water damage.
Mic issues on speakerphone and/or siri not hearing you can be remedied by cleaning the earspeaker mesh. If that doesn’t work, replace front facing camera ribbon. If its an issue during non-speakerphone calls, clean mesh on bottom of phone near charge port. If that doesn’t fix it, replace charge port ribbon. Soldering a new mic on is technically an option, just not entirely practical, especially if you aren’t experienced in soldering.
The ribbon is EXTREMELY thin and fragile, certainly much more so than its predecessors. I would try replacing it and make sure your connectors are nice and clean. The ribbon can be damaged, but not broken, leaving partial functionality. It’s not common, but it can certainly happen.
I’ve seen a number of devices that have hardware stop working after upgrading to ios 12. Batteries are most common, but if it happened after the 12 update, I would NOT anticipate that a restore would return functionality. Take it to a reliable repair shop with some integrity and you should be out less than a hundred dollars and the repair is quick, or if you’re comfortable/daring enough. buy the part and follow one of the guides for repair. If you see anything that looks like chalk/toothpaste on any connections, clean VERY well with alcohol and swab. I mention this because the number one reason I replace customers front facing camera is because the earspeaker area got some moisture in through the mesh and doesn’t kill the whole phone, but certainly causes issues.
If you have a screen available, try it out in a new screen. The ribbon that runs from your button to the board is part of the screen (unfortunately). That ribbon being damaged would actually be the cheapest fix, so hopefully that helps. Replacing the button or the board would be expensive.
I work at an electronics repair store, most of the time the solution is either a new sim card, or reset network settings (you'll have to reenter wifi passwords, but your phone will probably work, worth it).