Nice boxes.Don't over think it Mark.
Here's some candle boxes I made - the sides are all 10mm
View attachment 191649
Perhaps they needed to be 10mm as they have a sliding lid but even if it wasn't I'd still probably not go too much below.
Are you going to use hinges? If so that will determine the minimum thickness.
You'll be okay with dovetails at 6mm. Not sure I follow what you mean by mitred edges? you could do a mitre joint at 6mm but it won't be very strong - it just depends what the box will be used for as to whether it's suitable. I use 6mm mitre joints for jewellery box trays and they are fine.
Dovetailing with softwoods can be a bit of a pig. Cherry or Sycamore are (imho) two relatively inexpensive hardwoods that are easy to work with hand tools when starting out. You may find a local timber yard with an offcuts section?
On sizing I find having the golden ratio (1 : 1.618) in mind comforting. That may say something about how my mind works than anything else though!
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