What is the solution to stop fuel inside starship collecting in center of the tank during zero G flight? Here is a rough sketch of starship with rudimentary diaphragm system to control flow of fuel while in zero G:
In practice it's not worth preventing in large fuel tanks. The fuel is simply assumed to have floated away from the engines and any attempt to light them will at best fail (and at worst blow up your rocket due to uneven feeding.)
Rather, you use a much smaller rocket (could be the reaction control system) which actually does have bladders to prevent that from happening. You burn just enough to get the fuel in the main tanks to go to the engines, then you light the main engines. Alternately, you have some "small" (they can actually be substantial--I'm not finding the size of the ones on the second stage of the Saturn V, but the third stage ones were 3' tall) solid rocket motors to do the same thing.
The term you're looking for is an ullage burn.