Just wanted to give a quick update. Today will be the 4th treatment following the protocol Colu suggested (along with increasing the water temp to 81 degrees)
One of the corys passed yesterday :(
Today is the first day where I feel like things are actually improving. The corys no longer have excess slime coats, and I am not seeing any more of that fuzz on any of them. One of them had lots of fuzz over their face/eyes yesterday, but it is gone today :) They all seem to be doing a lot better, still resting a lot, but slightly more active, and I have been able to entice them to eat by soaking their pellets in garlic juice and letting them have lots of time to eat. Most of their fins (tail and dorsal primarily) are still looking ragged, but aren't getting worse.
The tetras and betta both developed more Ich spots, but there don't appear to be new ones today. I realized this morning that I had accidentally been under dosing the Ich-X, for at least one day and possibly all of them - I ordered some new droppers to help out with that. The tetras and betta are still eating well. The betta seems a little slower and is definitely resting more during the day (or I am paying more attention and noticing how much he rests now).
One thing that I think made a huge difference was going back and watching videos on how to properly gravel vacuum (the Aquarium Co-op one in particular) and holy moly I have not been doing it well for a couple months and it made a big difference last night.
As a newbie, it's been really hard to deal with this, so I wanted to share some progress for anyone else that finds themselves in a rough spot. Right now, I really think it can get better, I really hope so anyway :) It has for sure been discouraging and really tough emotionally, but hopefully it will get better.