That's not how Hex works, though it's an understandable mistake. Hex applies disadvantage on ability checks, NOT saving throws. Skills, basically.
The Shove action is an Athletics (Str) check vs an opposed Acrobatics (Dex) or Athletics (Str) check. So if you plan on shoving people, you want to hex enemies' Dex or Str - whichever is better. If you think an enemy is going to shove you, hex their Str. *Note: this is how Shove works in RAW. It is unclear if Larian implemented it in exactly the same way...
Wisdom is typically another good option, as wisdom governs Perception checks. So if an ally is trying to hide, hexing an enemy's Wisdom can help your ally stay hidden.
Finally, spellcaster ability checks occasionally come up. E.g., the spells Dispel Magic and Counterspell (both 3rd level so not in the game yet) call for Spellcasting Ability Checks to affect higher level spells. So you'd want to hex whichever stat the spellcaster uses, but only if you expect them to use these spells AND you're casting level 4+ spells.
tl;dr: Given the prevalence of Shove in BG3, you probably want to hex enemy Strength or Dexterity. Hexing Constitution does nothing.
