Has anyone ever had success fixing a water damaged phone by removing the motherboard, letting it sit in 99% isopropyl alcohol for 2 days before drying it out, and re-installing the motherboard?
I just tried over the weekend on two iPhones, without success. They were an SE 1st gen and a 6s, both used by relatives who were careless. The 6s had pictures that were otherwise not recoverable. Granted they were both dipped in salt water, so I wasn't surprised.
Ouch! Unfortunately, you called it: after immersion in salt water the corrosion starts almost immediately. I am guessing there was a delay between their mishaps and you having access to the phones. 
The SE was damaged pre-COVID, by my father-in-law. A wave hit him while he was on a ship touring the Antarctic coast. (There's actually a very interesting story how he took one of the last flights out of Argentina). I didn't get the phone until late last summer. It wasn't a big loss because his pictures were saved on iCloud. Plus, the phone itself was bought for real cheap - it was misadvertised on eBay as an iPhone 5c. Bidders also shied away from the fact that it was issued in Germany (though unlocked). I thought I'd dip the board in with the 6s board just to see but I had already pretty much written it off.
The iPhone 6s was more recently damaged on a river that flows into the Chesapeake. Unfortunately, I didn't realize that one of the motherboard screws requires a special tool, so that delay might have killed it.