https://ayumilove.net/files/games/raid_shadow_legends/guide/droprate/Dungeons_LootTable_DropRates. Drop rates for normal. According to Points per Energy (P/E) Level 16 was actually the best for Dungeon Divers with Level 20 for tournaments.
I havn't seen Drop rates for hard but since Hard 1 is same price as lvl 25.
I'm guessing maybe start by adding .15 P/E in Dungeon Divers and .12 P/E in Tournaments for Lvl 1
Math involved : Presume Mythical drop rate starts at 3% like Legenday did. In Dungeon Divers Mythical artifacts are worth 5 points more than Legendary . Then (5x.03=.15) So at 3% drop rate we can expect at least .15 P/E in Dunggeon Divers. This is without subtracting because there will be less lower tier artifacts. In Tournament Mytical Artifact gain you 4 extra points.
EDIT.... Of Course that is Assuming you get Mythical artifacts starting at level 1. But I don't think Plarium would be that nice lol... I will cut this out if I ever get Mythical at lvl1 hard... I am farming there only because I am trying to make dundeon divers and don't wish to waste energy.