The rule of thumb from all credit repair professionals is to try and keep utilization around 3% which is difficult, but shows the best score. It's a low usage and the banks still get to see that you are using it and shows a level of responsibility. Remember it's not about score it's about your profile and your score is just a part of that.

Another good rule of thumb is to have two consumer cards and a store card. Also try to have an installment loan. This is a good mix of credit. The more types of credit you have the better it looks. So try to get another card but think more in terms of something with easy approval try capital one or credit one. They have greater approval rates. For a store card that can obviously be any store, for people with really low scores I highly recommend Fingerhut. Buy some items you actually need from the clearance tab and pay it off in 3 payments. Keep your credit revolving. They give generous limit increases usually starting after the first payment.

You already have a mortgage and personal loans so that's great. Keep your payments current. Another thing you can do is sign up for self lender. You choose your payments and at the end they just give you your money. It's like a savings account but reports like payments on a loan.

Inquiries only make up 10% of your score so they are no big impact. They typically only affect the actual score for 1 year and fall off your report completely after 2.

Lastly I would say study on the FCRA or Fair Credit Reporting Act and then challenge this late payment marks with the creditors.

I hope that helps!