here is an idea for the 321. Hold off on the predeparture until boarding is near completed. Then once the door is closed, prior to pushback, 2 FA's execute a cart service in 1st (cart can be prepped before boarding. once the cart passes, collect trash and stow.
This may not be the ideal, but it utilizes the crew. while the 2nd FA is assisting, the 3rd & 4th can be securing the main cabin for departure.
Again, it may not be as "classy" as a tray svc, but it is efficent, and the premium customers get their pre-dep bvg.