I think you're worrying too much, @CreditGameGB.
The Flagship has tougher underwriting, the secured card is easier. All their other cards seem to be equivalent. I think you should just choose the card that is best for you.
WIth no rent, your DTI will be very low. Sounds like utilization is low as well. Your scores are very good. You have a reasonable age of credit history. They do care about relationship a bit, but it shouldn't matter that much in your case. All those accounts you opened are probably overkill. Most likely, they'll look favorably on you.
I only had 6 months credit history when I joined, and I immediately applied for a card. They gave me the More Rewards, and 9 months later my CL was almost $25K, and at a year it was at the lowest APR. I just have checking and savings, nothing else.
I don't know if they'll give you a SL over $6K. Income doesn't seem as important to Navy as it does to other lenders, but it's still part of the SL calculation. But whatever limit they give you, you probably won't get a higher one elsewhere. And if you start with a low limit, wait 3 months and request a CLI. Then you can request another, every 6 months thereafter. They're soft pulls, if you do them online or through the app. The max these days for a CLI seems to be 8K, or x3 your SL if lower. And after 1 year, you can request an APR reduction. You'll eventually hit the limit of unsecured credit they're willing to extend, but without negatives, it should be reasonably high.
