The OP seems happy with US, which has lots of service from DCA. Switching totally to UA for the (single) *A program would make sense only if the OP travels enough to benefit from getting Y+ for free on UA. Otherwise, a lower tier elite is likely to be treated better on US and get a few upgrades, although DCA will be tough. Also, I would avoid UA until they clean up their merger mess. (When and if this ever happens, OP can re-evaluate whether to use US or UA for his/her single *A program. Collecting in two *A programs at this OP's level of travel makes no sense.)

For the second program, do not pick another *A carrier. Either DL or AA would work, although if AA merges with US, the miles would be combined but the OP would again need to find a secondary program. I normally would like AS for the flexibility to collect either DL or AA miles into the same account (with little international travel, alliance partners of DL or AA are less relevant), but in this case, with AS having IIRC a single flight daily between DCA and the West Coast, redemption possibilities will be constrained by the AS rule that you may only use a single partner plus AS on award tickets. The OP needs to think carefully about award ticket routings of interest to evaluate whether this restriction on using AS miles would matter much.