I wonder, how do I know how solid/thick the wood has to be and how many braces there have to be? Is there a rule of thumb?
A known system of classification of wood is:
Hardwoods, which are flowering trees like, oak, maple, cherry.
Softwoods, which are conifers. Like pine, Douglas fir, redwood.
Generally, hardwoods are more durable and harder but there are exceptions.
Furniture used to be built with maple, oak, walnut, birch, and cherry, as well as pine.
These days because of the scarcity of good hardwoods They are used in combination with lesser quality wood and or used as veneer or laminate.
This is a useful guide to furniture wood, A guide to furniture wood
The choice of wood and design of the furniture go hand in hand, if you have a piece with a slender member subject to heavy loads you want to use good quality oak or cherry, or other high-quality hardwood.
Then you narrow your choice by the color and grain and stain of the wood.