I agree with
@Buddy the science is clear on how to interrupt the lifecycle of ICH. 70 days fish free, and tank transfer method or copper treatment. You can be ICH free by maintaining a strict quarantine procedure for each new addition.
I have been through this before and the sooner you get those fish out, the more success you will have at having them come through. The hardier fish with thicker coats will survive, while the rest will be overcome and die.
The misconception that 'just leaving things be' is an adequate treatment is damaging, and your two experiences should be an example of that to everyone reading this. The fact that hardier fish will do okay and appear ICH free is what perpetuates the misconception. Your tank will continue to reinfect new fish as you add them (as appears is what has happened) unless you interrupt the lifecycle of ICH.
UV does not work. It's hard work to do 4 tank transfers (one every 3 days) and then keep your fish in a QT tank while your display remains fallow for 70 days (
minimum). You have to bleach, rinse and dry
all your equipment between each transfer so you need two sets of pumps/heads/airstones etc. I did it when I had ICH and it worked. Fish have remained completely healthy and once I got the fish out of the display, I had no further losses.
I'm sorry for your pain and suffering but you need to do the best thing for your tank.