Carrying a balance on your credit card so that you owe interest does not increase your credit score. Period.
I don’t know what you mean by leave 30% of your available credit on the card. Do you mean leave unpaid after the statement due date so that you owe interest? Because that is absolutely terrible financial advice with no upside. You should keep your credit utilization under 30% and probably under 10% - i.e. your statement balance vs available credit. This should be paid off in full every month by your statement due date.
You had a loan that wasn’t paid off and it hurt your credit? Big surprise.
