Ok, so little steps, I have divided my model up into its component parts. Individually, each of these can be imported as a glb file and work fine.

however

If I join more than three or four of these together, I get the file error in Powerpoint. This makes me think that perhaps Powerpoint has changed the maximum size or complexity setting that it can import.

Is there a way of getting a stand alone, earlier version of powerpoint to try?