JohnnyL welcome here! I myself haven't been on this forum for that long myself but am glad I joined, both for the knowledge and moreso the conversation in general with people as crazy (or even far crazier) as I am about the little details and the science of handloading and ballistics - most of the people I actually shoot with or talk in person with aren't all that interested in the kind of things we love to talk about here, only one of my friends even reloads.
I have no .257 experience or Eld x experience so my opinion is nothing but a hunch...
I'd be super curious to see what IMR Enduron 7977 does with your cartridge and bullet.
I see you're a fellow Canadian, so you'll especially see the appeal the temp insensitivity (though I've heard h1000 is good for that too) but more than that I've heard some complain that the eld x bullets in particular is bad for copper fouling quickly - 7977 has a copper fouling eraser in it.
25WSM suggests 7828ssc is best and he DOES have a ton of experience with the round so I'd say listen to that...and know that 7977 is meant to approximate the burn rate of that great powder - and is comparable to h1000 too - (it is a tad slower, I do have experience with both powders in .270 and .300 win) while being able to exceed its max FPS at equal chamber pressure (double base) with temp insensitivity and copper fouling erasing. So that's my welcome and my hunch...try the 7977!