Phone-first, because that is the only screen that matters
Almost nobody opens your invitation on a laptop. They open it on a phone, often one-handed, sometimes in bright daylight or on a slow connection. So the page stacks its blocks in a single clear column: the guest scrolls rather than pinching to enlarge, and the reply action sits where a thumb naturally reaches, not stranded at the bottom.
Times and addresses stay legible at a glance, and nothing on the page demands a download or an account just to say you will be there. On a small screen, a landing page that behaves is worth more than one that merely looks fine on a wide monitor.