So I went for a dumper truck. Still a favourite, still easily recognizable, still a truck - but could be any colour I want, no writing needed, and somehow felt like it would still look ok even if the shape went a bit wrong.
The cake was this one, baked in a loaf tin. I cut it into three pieces: about a third from one end to make the front part (standing on the cut side), all the rest except a couple of inches to make the back part, and then the last piece to cut into six wheel shapes. Then I shaped the front, cut out the hopper, shaped the wheel beds and evened out the wheels.
The icing was this one. Instead of cinnamon, I added a few different powders to colour it: the icing on the back part of the truck is with freeze-dried raspberry powder, the icing on the front part is with acai powder, and the icing on the wheels is with cocoa. I considered a few other powder (spirulina or matcha for green; turmeric for yellow), but then realized it took quite a lot of powder for a get much colour and I more wanted it to taste nice...
The decorations are: mango-coconut balls for the load and lights, dried kiwi slices for the wheels, and raspberry-apple fruit 'spaghetti' for the writing, window shapes and stripes on the back part. It would have been best presented straight after decorating, but it looked ok the next morning.
It tasted good but yet again I made a sage birthday cake that failed to rise properly (it rose beautifully, and then dropped a lot). This time I had even decided to follow a proper cake recipe to the word to prevent this... Oh well, the icing works!
And, most importantly, S liked it and recognized it as a dumper truck. He got a yellow dumper truck from the 1950s from my dad so it all fit together perfectly.