Still being newer, this is something I wonder myself. I think right now I lean toward openings.
If you had to choose one: opening theory or endgame technique?

Endgame technique will win you more games (and will also save more lost positions) than opening theory will.
Opening study is more fashionable.

Playing against someone at my rating, I'd feel very confident playing against someone with tons of opening theory and poor endgame skills. I'd be nervous if it were the other way around.
Opening, you don’t need to worry about endgame until you get to it. If you can’t play a good opening you won’t need to worry about endgame

Opening, you don’t need to worry about endgame until you get to it. If you can’t play a good opening you won’t need to worry about endgame
There are probably a lot of good analogies. Maybe a race. Which is more important, starting the race strong or a strong finish.
I would say crossing the finish line technique doesn't matter if starting technique is lousy or even average. I'll bet most competitions are won by the person who has an early lead and maintains it. My guess is come from behind wins at the end are far more rare.

Endgame theory of course.
Openings are a PITA.
Being a pita is testimony to why they are more important.
I don't enjoy studying either but I think openings would be far more beneficial than endgames.
Playing poorly in the opening can lead to worse positions in the middle game, which will end up giving the opponent a clear advantage in the middle or end game (bad opening most likely mean you lose).
On the other hand, outplaying your opponent in the opening means you have an easy job for the rest of the game. Of course this probably only applies from 2000 up to 2400, where openings can usually decide games.
Exactly. Openings are the foundation. A poor foundation is going to result in a poor game. Someone who doesn't play openings well doesn't have to worry about endgames. They will just resign before it gets that far.
As long as you keep a relatively even game in the opening you're fine... it's not that important to memorize opening lines if you can play the right moves (even if they're not the engine-approved objectively best ones)
I agree 100%. It's not that important to memorize opening lines. Memorizing them (or any other part of chess) probably isn't going to help much. But understanding them will help.
People who memorize openings but don't understand them usually end up having difficulties after about 10 moves or so. Which usually means difficulty in the middle game. Which usually means endgames don't matter. They resign before they get that far.