Sounds fun.
I think the only thing is that it’ll be hard to pick out enough detail in mainly the shadows on his arm for it to look decent. Other areas are quite detailed too and pretty important imho
In your mock-up, for me, I don’t think there is quite enough detail to recreate the picture properly.
Too little detail and it might get lost a bit?
I wonder if you could combine a little pyrography here and there to bring out detail in areas where it’s otherwise difficult to get with the wood itself? Blending it into the stained timber.
French cleats would work on the bigger pieces but might be a faff on smaller sections. I’d get a headless pinner and wang some of them in.
Also, when you get your timber, it might be good to put it in the room for a couple of weeks or so to acclimatise and reduce the chance of shrinkage widening gaps and that.
Photos of progress please.