I ganached the cakes last night and won’t be needing the dummy after all, as I did a 7" cake baked in a 6" tin and also for the dome. So after I layered and ganached them it will stand approximately 10" when stacked.
Thanks for the advice on the smaller board between the top cakes, I cut smaller base boards and used those.
For the fondant I’m going to wrap the dome seperately, and then use the wrap method for the bottom stacked cake.
There will be fondant decorations and a strip going around near the top which will cover the join between the dome and cake.
Fingers crossed all goes according to plan.