I have a True Draco like Mariamune destroy Guardian Eatos and another True King (maybe Bahrastos) in my hand, to summon itself.

It summons itself, and the other True King it destroyed is always chain link 1 if I choose to activate it at all, while Guardian Dreadscythe can only be chain link 2 (or chain link 1 if I don't want to use the other king's effect). This kinda sucks since that causes the scythe search from to miss timing.

So, should Dreadscythe always be chain link 2? I could find online that trigger effects like Dreadscythe that summon themselves from the hand (presumably also the deck like with Theinen, they're both not public knowledge anyway) can only be once per chain, but do they have to be a later chain link? Otherwise segoc seems to indicate that I pick the chain order from two optional trigger effects on my own monsters.

Enable auto chain order in the settings.

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First of all, it needs to be unchecked in order to pick your order manually, not checked.
and second of all that doesn't change anything anyways since Dreadscythe is still forcing itself to be chain link 2. the effects are, for some reason, not considered to be happening at the same time in the simulator, although Mariamune obviously destroys both simultaneously