Choose a role before investing
Decide whether a hero will absorb damage, heal or control, or deal damage. Then check base stats, preferred weapons and skills. Rarity alone is not a reason to spend every book on a hero who does not fill a useful role. Use the skill reference for effects and the hero reference for starting stats.
Read trait-changing item descriptions
Not every trait-changing item only adds traits: some methods can change or remove existing ones. Direct use, supervised reading and different books have different rules. Read the current cost, outcome description and retention options before confirming. Old probability claims are not a universal guarantee.
After a clear, check what changed
Later runs change enemy capabilities and goals. A loadout that worked before may need revision. Check the current opponent and newly available growth or challenge menus before choosing more research, a different team or further training. Follow the in-game schedule for arena and guild activities.