Clear speed before niceties
Creator guides confirm the priority: spend gold and gems on the upgrade that lifts your wave clear — a unit, a star level, or an evolution — before pushing optional rolls.
Every gold and gem you own has one job: make your next clear faster. An upgrade that shortens your farm runs pays for itself by raising everything you earn per hour. Anything that doesn't speed a clear — a cosmetic form, an extra bench unit, an unneeded roll — is a nicety you fund only after clear speed is handled.