I would prefer that no Mythic tokens makes existing Legendary or Eldritch Relic tokens obsolete, with the exception of Treasure Enhancers, and potentially mainhand/offhand items.
By obsolete I mean there is no reason to equip the existing Legendary/Eldrtich token any longer, given the existence of the new Mythic token.
For example, the proposed Mythic +10 STR waist slot item will make Legendary Surtr's Girdle of Fire Giant Strength obsolete, since there is no reason to equip Surtr's Girdle if you can equip a mythic +10 STR belt.
To avoid this outcome, there are only a few slots to be concerned about: waist, neck, torso, finger, feet or wrist slot.
Every other slot either has no Legendary / Eldritch Token, or has/will have a way to equip a Mythic as well as every existing Legendary / Eldritch Relic tokens.
I'd focus Mythics on slots that either have less contention, or do not currently have a contested Legendary or Eldritch Relic token, e.g.:
First go after Mythics in these slots: Back, Bead, Charm, Ear, Eyes, Figurine, Hands, Ioun Stone, Shins.
Then go after: Head, Legs, Shirt (which will all gain the capacity to have a free slot with the Safehold program).
For instance, I believe a +7 STR gloves Mythic would be about as desirable as a +10 STR waist Mythic, and wouldn't have the side effect of obsoleting a Legendary token.