Hi @NeilSpinney
Last time we prepared a modified version of the MainWP Child plugin that synced 10 000 post categories from Child Sites. And when the new version of the MainWP Child plugin was released, the modification made got overwritten, and now the limit is back to the default value of 300.
We will make a filter that will enable you to customize this number of synced post categories.
As soon we have it ready, we will share it with you via this thread.