Would a Christmas release been better?

I don't think so.

I think that a big reason why this movie failed was because its a forgotten character, I can't think anyone under 30 genuinely excited for this movie.

Me in particular, I skipped this movie because of the spoilers regarding how the character was written for this installment - I did not want to watch one of my childhood heroes as a fragile old man.

No, the movie had always failed. I really wanted to like the movie, but it is just mediocre. It is better in general as Crystal Skull, because it does not have scenes like Mudd as Tarzan. But Wombat is as interesting as Mudd, the movie does not feel often like Indiana Jones and formost too many mediocre way to long action scenes. Not sure what went wrong, but Mangold could have kept the budget easily reasonable, if Tangier had been cut completely and New York and the Intro cut down drastically. Then the movie had been more reasonable 100 Minutes and likely 100M less in costs. Easier to position, easier to attract people and at least the old fans wouldn't be bored out. Only then the movie would have had a chance to not flop.
As it is, no chance, more as projected 500M was always imo questionable for a series, which spark absolute no interest in the current main box office target group and the damage Crystal Skull did to the franchise.

A better movie would have been better.

I think they lost a huge chunk of Indy fans because of Skull. A shorter run time would've helped, but not much.

Indy died in 08.

I don't think it was competition that hurt the film. Look at nearly every other film that's been released of late. With only a couple exceptions, all films are underperforming at the box office. During the Covid lockdown, people became accustomed to streaming films at home, and studios acquiesced to their, in my opinion, short-sighted laziness. Movies seem to appear for "free" on streaming platforms very soon after they hit theaters, sometimes even the same day.

On top of that, at least in the U.S., we're facing inflation levels far higher than at any point in anyone's lifetime, and a stagnating economy. People are spending less on entertainment, and a trip to the movies will be one of the first things to go now that other options exist.

Look at the 2023 box office numbers and you'll see that films that would have brought in massive profits-- Transformers, Mission Impossible, Fast and Furious sequels, two MCU films, the Flash teamed up with Keaton's Batman, and now Indiana Jones and Oppenheimer-- all selling far fewer tickets than they would have, pre-pandemic.