You first need to decide what you want from all this.

Real airline miles have the most value when there is a lot of them pooled in one account. They have a lot less value when spread around a lot of accounts, none of which have signficant balances.

Therefore, since there are three alliances, even if you choose among more than three airlines, you should probably have one account with an airline in each of the three alliances, and US Air takes care of Star. The only reason to stop collection Star Alliance miles in US Air and switch to United is if you wanted to go for upgrades (or if your further research showed that redeeming US mlies for UA flights is a lot harder than redeeming UA miles for UA lfights). But you'll not likely ever got to the point of upgrades unless you mostly stick with one alliance (and have the minimum number of flights per year on the carrier in that alliance with whom you collect).

Real arilne miles have the most value when used for otherwise expensive trips. 25000 miles for a coast-to-coast trip that you could have bought on sale for $300ish is considered by most a poor value. 25000 miles for a LAX-JAC (Jackson Hole WY) trip, even though way shorter, that would have cost $500-$600 or more cash is a much better valuel. 90000 miles for a LAX-Europe trip in business class (if you can appreciate that), is by far the best value, because business class from LAX to Europe will cost several times more than economy class, yet you only need about double the miles to get a business class international ticket as an economy class ticket.

OTOH, you would mostly use miles for flights that are not that expensive anyway, then what you want is not a "fake miles" card like Capital One but an oridnary cashback card. See, all those "fake miles" cards do is collect points at the equivalent of about 1 point per dollar spent and then turn those points into tickets at the cost of about 1 dollar per point. If you look it closely, that's exactly the same as a 1% cashback card, except with a 1% cashback card, you don't have to wait until you've spent $25,000 before you can get anything from it.

But nothing other than "real miles" will let you "multiple" the value you get, because only with real miles do you get chances to get flights that would have been much more expensive with cash. Neither cash back nor "fake miles" would help you with that LAX-JAC ticket that cost $500-$600 and up with only $25,000 of credit card spending, but real miles (at least in the case of UA) would, in my experience. (I don't have US miles so I have no idea how easy it is to get such a flight with US miles rather than UA miles.)