1x Tau-Forged Crimson Archon Shard for Ability Strength +15%, which gets us to 272% for Gloom's 95% slow cap.

If you swap Primed Sure Footed for Power Drift, we can replace this with 1 more Azure Archon Shard for Armor.

2x Tau-Forged Crimson Archon Shard for Ability Duration +30%, which get's us to 97.5% for Gloom to drain 0.3-3 Energy per second based on enemies in range.

If this feels like too much energy economy, you can replace these for more Strength or Armor. I recommend Armor as we have already hit the hard cap on Gloom's slow and Strength isn't Saryn's strongest stat. However, it does give us more damage on Toxic Lash and more Life Steal on Gloom.

2x Tau-Forged Azure Archon Shard for Armor +450, which gets us to 1327 Armor for roughly 81% damage reduction.