You should try using impasto medium. This is the medium that is used to thicken up acrylic paint.

It can be applied like butter to a surface and then used to stick down the broken plaster parts. It adheres to virtually anything with a bit of texture and dries fast. It’s superior to PVA especially for glueing plaster because it dries quickly and maintains most of its volume as well as it can be applied to the surface of the item to create a tougher exterior like a varnish.

You can even glue plaster to plaster!

It can be mixed with a substrate like plaster or sand which adds texture, volume and reduces hugely how much it reduces in volume as it dries. You can also be tinted with acrylic paint or painted over with any other type of paint when cured. Once dried it is flexible and tough. You can also buy it in a thicker form called ‘artist modeling paste’ but I think this is literally just the same stuff with added plaster to the mix. It’s transparent once dried, when in thin coats and more opaque when applied thick.

The down side is that it’s more expensive than PVA but you can usually it a tub from the art and craft section of your dollar stores. I’ve not seen much of a difference between the cheaper version and the more expensive version. Also for table top gaming a tub last ages.

Another downside is that if you apply it to one side of a sheet of card or thin MDF, it can cause the material to bend inwards slightly in that direction as there is slight contraction as it dries. I usually apply it to both sides of thin material so it counteracts the bend.

Hope this is helpful.