Where should I invest so I lose nothing at the end of the Reign?

Put your resources into the permanent, never into the disposable late in a Reign. The permanent crosses the reset:

• Stars (Star Rank) — via Relics
• Habits — via Breedmarks + 🔵🟣🟡 Cores
• Relics, Relic Shards, Breedmarks, Cores, keys — stored, never lost
• Unlocked Heirlooms (plus the always-active Mastery bonus)
• Gold, dragons, eggs

Conversely, at the very end of a Reign, avoid over-investing in: buildings, research, dragon level, base resources and Meat (all of it resets or is lost).

A transition tip: just before the Reign ends, spend your remaining Meat (it disappears) and convert what you can into permanent gains. At the start of a Reign, re-grind your House Level fast to reactivate the Heirloom buffs you've already paid for.