It's hard to give you advice when after trying to do so you add all sorts of facts like the above which may completely change the equation!

Originally all you said was you wanted a free flight a year LAX-BNA. Later you added that you flew additional times, but I think you said your flights were paid for by your family in those cases.

Who will pay for the flights to Tampa and Detroit? (Presumably not you, because if you could afford all those, why would one more to Nashville be a problem?)

There's a major fact which I'm not sure you realize that's the key to why business people are a major user of FF programs: The person who gets FF credit/miles for a flight is the booked passenger, NOT the person who paid for the flight. So every paid (paid by whomever) flight you make to Detroit, Tampa, and Nashville earns you FF credits/miles. (But that's only of great use if you use the same airline in all cases, or at most maybe split it between two consistent airlines.)

Furthermore, Southwest goes to all those cities from LAX. 6 of those flights with NO credit card earnings at all will earn you a free "anytime last-minute" flight to Nashville. (On any other airline, it's likely to take dozens of flights to equal one free flight.)

If you only need one free flight a year and you earn that by all the paid-by-whomever flights you take the rest of the year, why worry about any credit card at all? But if you are going to fly an airline consistently to all those cities and earn each time you fly, and you still want to earn more with a credit card, it only makes sense to earn with a credit card that puts your miles/credits into the same FF program you're earning miles/credits through flying, because then your flying earnings add to your credit card earnings and produce free flights faster.

But if you have more facts (are you going to stay in hotels? are you going to rent cars?) it's time to put them all out now, because each such fact can again completely change what solution makes the best sense.

Btw, if your GF is going to fly "a lot" to Tampa make sure she signs up for the FF program on the airline y'all fly to there. Since she's a different booked passenger, she has to have her own FF account. Again, it doesn't matte who buys the ticket, it matters who flies in the seat on the plane as to who earns in an FF program. So you earn for your tickets, she earns for her tickets, no matter whether you, she, or someone else pays for all those tickets.