Hi KG6819
Aww bless, I see your problem, there are many reasons why a cake will sink in the middle but I need to ask first if you skewered the cake first before you pulled it from the oven? As looking at the picture the cake looks slightly raw still.
From your pic the cake has baked on the outside but not the middle so I'm assuming that you didn't open the door whilst first baking the cake (as this would add a mass amount of cold air into the mix and ruin the science process) then I would suggest you need to re bake a 6 inch on a lower temp of your oven and cook for longer. Deep cakes take longer to bake so it's always best to lower the temp I usually bake my deep tins on 150°C -160°C. My 6 inch round tin can sometimes take me up to an hour to bake.
As your 8 inch is coming out fine then I would rule out your oven temperature being temperamental but other causes of sinkage that can cause issues are also as follows:-
Baking 101
Make sure you use a sturdy recipe for a wedding cake something like a Madeira as they are strong and can take the weight of tiers too and check your recipe also making sure that you've added all the ingredients and or are using the correct amounts. Raising agents can also cause sinkage so ensure you use the correct amount if you are using some.
I do hope when you try the next one it comes out fine, unfortunately even as experienced bakers sometimes it can all go to pot but I'm sure you will be fine.
If you need anything further then please let me know.
Good Luck.
Angie x