If you have a United card, and the only flight available somewhere is on another airline, and you charge it to the United VISA, do you still get miles toward a future United flight?
Also, I see that some of the United cards give free check on bag allowance. Do you know if any of them give any boarding order preference?
I am pretty confident every major airline card except for Jetblue give a version of priority boarding if the card comes with an annual fee. May not be the best priority, but will be better than the average pion.
Don't confuse people who fly a lot and and have status with credit card spending. You aren't traveling enough and it doesn't appear you are spending enough for it to be relevant. If you get an airline specific card you should be doing it for perks such as free checked bag and priority boarding, not looking to put all your spending on the card.
Depending on your spending levels and desire for priority pass and travel insurance, a Chase Sapphire Preferred/Reserve with a Freedom Flex Unlimited would be a two card combo that would cover all your bases. You could transfer the point earned to united if you wanted, but depending on flight you would be better off using the points through Chase's travel portal/statement credit rather than transferring directly to united.
If you are looking for just one card that would give you decent points, priority pass, reasonable annual fee, decent travel protection than I would look at the Capital One Venture X. Annual Fee is $400, but with $300 in travel credit and a 10,000 point bonus every anniversary year it is essentially a $0 annual fee card. $100 Global Citizen credit would put you net ahead.
Another option to possibly look at is Citi Premier. Has an annual fee of $95 and doesn't have priority pass, but it has a good sign up bonus (80,0000 after 4k in spend) and earns 3X Thank You points on Gas, Food, Travel, Supermarkets. Biggest benefit here is earning 3X on so many everyday expenses--some people just really don't want to deal with the hassle of using 2 or 3 different cards to maximize their earn rates. I prefer being in the Chase eco system, but I sign up for this card every few years for the signup bonus. It's not a bad option since you already have a citi account.