Do some research on what you could earn in a similar position with another employer. Make your case to your current employer about the fair market pay for your range of responsibility. If they don't meet your expectations then be prepared to leave.

I wouldn't recommend waiting around for multiple years in the hope of replacing someone else - if you're underpaid now, your employer will likely just low-ball you in the new position as well.

You can build credit without carrying a balance. Keep your credit utilization low (less than 30%, even better if you can do less than 10%) at the end of your billing period. You may want to make mid-month payments to achieve this.

Request a CLI at whatever interval the card issuer will allow you to (usually 6 or 12 months) - that will also help you keep your utilization low.

Spending money you don’t currently have in your checking account is a slippery slope and will likely end up hurting your credit in the end.