Hi Alex, you could definitely make the Victoria sponge into a chocolate cake instead by adding in cocoa powder (about 80g should give it quite a good chocolate taste, but my only worry would be that it may make the recipe slightly more dry in texture by adding more dry ingredients and so a splash more milk may be needed).
As for the buttercream type filling/topping, what you have suggested will work well (any full fat cream cheese should work so Philadeplhia – or stores own brand), some double cream, cocoa powder, a drop of vanilla and sweetener. It will basically taste the same as the chocolate cheesecake topping: https://www.gestationaldiabetes.co.uk/recipe/chocolate-cheesecake/
I made a similar icing on the mocha cupcakes recipe too, but without the cream cheese: https://www.gestationaldiabetes.co.uk/recipe/mocha-cupcakes/ just add cocoa instead of coffee.
You can buy an alternative to icing sugar which is sweetener, but it is VERY expensive, but I wanted to add for anyone else reading who wants to splurge!
The other thing I should mention is that you can buy coconut flour in Aldi (most stores stock it near the nuts/seeds, healthy wholefoods type section) and you can also buy it online much cheaper than in many stores.
Anyway, hopefully that helps and you can create the perfect GD choc birthday cake that you can both enjoy. Happy Birthday to your little one! Jo xx