I have used that concentration on clown loaches once, I had been away and they were new and heavily infected, they were around 2 inches lond. Because I had heard that clowns were salt sensitive I added 1/4 of the salt every 12 hrs to reach the total, out of 8 fish 1 died. Considering how bad they were I found that acceptable.
As for the water changes I spot a problem im your logic, the water you are removing is a salt solution, salt is being removed with the water. Your new water should also contain the same strength solution of salt/water. This is not like when you are replacing evaporation losses which leave the salt behind.
1 teaspoon per 5 gallons is about 8-10 times less than what you need to be even halfway sure of killing ick. Diluting it with an unsalted 50% water change leaves at 5-6 % of the salt you need