The pertinent ruling is apparently
608.2g: If an effect gives a player the option to pay mana, they may activate mana abilities before taking that action. If an effect specifically instructs or allows a player to cast a spell during resolution, they do so by following the steps in rules 601.2a-i, except no player receives priority after it’s cast. That spell becomes the topmost object on the stack, and the currently resolving spell or ability continues to resolve, which may include casting other spells this way. No other spells can normally be cast and no other abilities can normally be activated during resolution.
EDIT: the above rule is why you can cast spells with Gix. There’s a ruling on Xanathar that says “Xanathar doesn’t change when you can cast spells […] You must still follow all normal timing rules”
Ahh, so in the rule it says “If an effect specifically instructs or allows a player to cast a spell during resolution” - Xanathar is instructing the player to cast a spell until end of turn, rather than specifically during the resolution of the triggered ability in the upkeep where you choose your target opponent. Makes sense now, thank you
Exactly, yep!