re: More practice of Veneer Hammering.

Toothing the substrate never hurts, especially when it is a “factory surface” that may have been burnished or contaminated like plywood can be. It will improve the bond. Toothing thin sliced veneer is not normally done and could cause trouble with sand through later on.
For my money those are not bad joints considering that they are very long miters and that wood expands a lot more across the grain than along it. I think the best way to improve the appearance may be to shorten the miters this could be done within the existing design by adding a band or string half way through the maple dividing the miter into two narrower pieces. That’s a late night top of the head idea. It may not stand up in the cold light of day but it seems to make sense to me at the moment.
I think you’re doing great.