One more thing: make sure to warm up the chocolate fudge sauce to make it easier to spread. Just 30 seconds in the microwave can make a big difference and this way it won't pull up the other layers as you try to spread it out.
I'd microwave it just a little each time you need to spread it.

Hi Michael, I would definitely suggest making the components ahead of time! I prepped all the components and baked the sheet cake on day 1, then I assembled on day 2, and then I served on day 3.
The banana compote did discolour when I made it. I really thought by cooking the bananas, they'd keep their colour, but that didn't work out as well as I'd expected and it definitely greyed with time.

I think lemon juice or a pinch of citric acid powder could help, as you suggested. I haven't tested it though. With citric acid, a little goes a long way because we don't want to introduce too much of a flavour. I think it's definitely worth trying.