How do I prioritize my spending knowing the season will reset?

The rule is simple: favor the permanent, never over-invest in the disposable.

To favor (permanent, kept forever):

• Raising your key dragons' Star Rank (Relics)
• Upgrading Habits (Breedmarks + Cores)
• Buying Heirlooms and their Mastery bonus
• Keeping your Gold for genuinely permanent levers

To treat as disposable (re-grinded every season):

• Building levels and research
• Your dragons' Reign Level
• Your stock of base resources

That doesn't mean ignoring the disposable: you need it during the Reign to fight. But don't bankrupt yourself on it at the end of a season. And early in a Reign, re-grind your House Level fast to reactivate the Heirloom bonuses you've already paid for.