Right-clicking a full composter with a shovel should drop dirt

Once a composter was full (level 8), right-clicking it with organic matter like seeds would still produce bonemeal as usual. But if you right-clicked it with a shovel, it would pop out a dirt block instead, before resetting to level 0 like normal.

This would keep existing gameplay and bonemeal farms working exactly the same while still giving the extra option to players that want it.

It's true in most games you usually have too much dirt rather than too little, but allowing renewable dirt (other than the wandering trader podzol conversion) is useful for island spawns or skyblock worlds. This method makes it possible while not being so easy/automatable that it removed the challenge and fun of those situations.

EDIT: Now that 1.18 is revamping generation, this would also mean underground dirt veins, which always seemed kind of weird to find deep down, can be removed without affecting the balance/availability of dirt for players. Clay or terracotta deposits could generate instead.