Looking very nice! Wood and aluminium together is just sooo easy on the eye :-) Interesting comparison of the wood and steel, just keep in mind that the force in the direction you simulated is actually very small in gantry routers. Because the cutter is located relatively far from the gantry the force on the gantry is more rotational rather than straight in Z and Y, due to the leverage created by that offset. Would be interesting to see the same comparison with that in mind.

I think the experienced guys on here might recommend two ball screws for your long axis to prevent racking.

I've often wondered about the way you have your rails on the gantry because it does sort of make sense to me. Not sure if someone has tried that yet but it might be a bit tricky to do the alignment. I guess the same applies to the way your Z rails are oriented, those Z parts will have to be perfect to get those rails parallel. For Z its also generally recommended that your carriages are fixed to Y and the rail to the moving Z plate.

I do agree with your comment that a lot of frames are way over designed but don't underestimate the importance of a good solid foundation for your machine. Some will likely disagree with me here but the stationary base of a machine can never be too solid. For starters, you don't want the whole thing walking around when its briskly moving that 77kg gantry during cutting:-) What you have there looks relatively sturdy to me but I'll leave it up to the pros for comment.