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)
At this point, opening theory. I never feel comfortable in the beginning of the game and usually find myself lost after the 4th or 5th move.