This will die before the triggered ability resolves and has memory issues.
Suggested wording:
"When you cast ~, choose a creature type. Each opponent exiles a creature of the chosen type they control.
~'s power is equal to the total power of creature cards exiled with it and its toughness is equal to the total toughness of all creatures exiled with it."
Rules might need a little adjustment regarding retaining memory of the exile effect from spell to permanent - alternatively you could set the power/toughness statically to the value it is as the spell resolves.
Much simplified version, used Sutured Ghoul and Diabolic Edict as guides.
Would it work with just "As ~ ETBs..."? It would still be fiddly but avoid the immediate problem.
Having the toughness be determined by an etb trigger seems like a rules nightmare, since its toughness will undefined until the trigger resolves.
Maybe make this an instant/sorcery spell that creates a creature token?
The toughness isn't quite a mirror; and is so rarely going to be anything other than 1. Could it be made a direct mirror of the power ability?
This will die before the triggered ability resolves and has memory issues.
Suggested wording:
"When you cast ~, choose a creature type. Each opponent exiles a creature of the chosen type they control.
~'s power is equal to the total power of creature cards exiled with it and its toughness is equal to the total toughness of all creatures exiled with it."
Rules might need a little adjustment regarding retaining memory of the exile effect from spell to permanent - alternatively you could set the power/toughness statically to the value it is as the spell resolves.
work in progress
First of a cycle of slivers utilising newer keywords. Rest of cycle is Trained Sliver, Corrosive Sliver, Heroic Sliver, Undying Sliver.