UW admissions notification is around 3/15.
Physics is an open major: “Students in good academic standing may declare the major at any time by visiting the department advising office to complete the necessary paperwork.”
Astronomy only requires a cumulative 2.0 average in PHYS 121, PHYS 122 and PHYS 123 (or full transfer equivalent) to declare the major.
OP is on the right track regarding the double major, as the Department of Astronomy states:
“Undergraduates interested in advanced work in astronomy are advised to take a double major in astronomy and physics. Undergraduates interested in immediate employment at an observatory or other scientific institution should include computing and electronics courses as part of their program. As a capstone sequence of hands-on research and dissemination of results, the following is highly recommended: ASTR 480, followed by either ASTR 481 or ASTR 499 or a Research Experience for Undergraduates (REU) project, and ending with ASTR 482.”