I do see why you want to increase your scores, since 715 is not high enough to qualify you for the best rates.
It's quite possible, however, that you are not looking at your FICO mortage scores, but rather some other kind of scores. (For example you might be looking at FICO 8.)
Are you certain that you know what your mortage scores are? If not, your first step should be to pull your mortgage scores, which you can do by getting a subscription to the myFICO Ultimate product here. You can cancel it after a week.
Actually, since the Ultimate product costs money, I'd first do everything you can with your cards to optimize your score before you pull your mortgage scores. Specifically I would make sure you have one plain credit card reporting a small balance (e.g. $20) with your charge card and your other cards reporting $0.
Do you have any closed installment loans on your reports?
What is the age of your youngest account? (Closed or open)
What is the age of your oldest account? (Closed or open)
Assuming you do not apply for any accounts between now and when you go through pre-qualification, do you have any inquiries that will become over 1 year old?
