There's no need to move him to another tank if he lives alone. In fact, it may only stress him more.
Ich is caused by some form of stress 99% of the time. Either something is wrong with the water quality, something in the environment is stressing the fish out (a tank mate nipping them, for example...or not enough hiding places), or he has another underlying problem.
So, with that being said, I'd test the water asap for ammonia, nitrite, and nitrate. You said the parameters were "fine" but actual numbers would help
. Are you using test strips? Those are notoriously inaccurate.
If you have freshwater aquarium salt, use that. If not (it's cheap btw), you could use non-iodized table salt. The dosage instructions for the aquarium salt are on the carton, if you use table salt I'd imagine you'd dose about one teaspoon of salt per gallon or something, keeping in mind that your tank really holds 3-4 gallons after decorations and substrate.
Why did you have a pleco in a 5-gallon? The pleco may not have brought Ich into your tank but simply stressed the betta out. I wouldn't get another one.
Other than the salt, just keep up with water changes and raise the temperature. What is the actual temperature right now? If it is 80 degrees, you could raise it up a few degrees over the next few days. That will speed up the parasite's life cycle.
And as you may know already, keep up with the treatment for the next few weeks even if the spots go away.