Upgrade for the current problem
When progress slows, identify whether the issue is clear speed, resource flow, or a missing unlock. Spend toward that problem first, then reassess after a few waves instead of assuming one category is always best.
Use systems as they unlock
Loot, jobs, crafting, alchemy, mining, Shaping, Masteries, and Guardian Perks are all named as progression systems. The supplied sources do not list their precise unlock rules, so this page does not assign unsupported thresholds.