There are two more rarity tiers after artifact but they already start appearing on 7 length words if I remember correctly. After that the longer ones only affect the dungeon size. For now at least. I will definitely keep on working on this game as I still got loads of stuff "planned" that I couldn't squeeze to the jam range.
Yeah now that I think about it, the hp percentage is kinda poorly balanced as the main bulk of your hp comes from flat bonuses (which scales with gear rarity) and that makes the non-scaling percentage boosts absolutely garbage in comparison.