I just want "a" to make the same sound it makes in literally every other spelling system, including Cyrillic and Greek, not "ey" or "eh". Seriously, that's one of the most stable phonemes, how did it turn into "ey" in English?

The vowel probably shifted in English and the spelling wasn't adjusted. "Wal" in German is pronounced... almost like you'd expect, except it's a long 'a' without any of the usual markers for that (aa or ah). German spelling is more phonetic than English but less than Spanish.

The Great Vowel Shift. https://en.wikipedia.org/wiki/Great_Vowel_Shift

Long story short you could say they rotated their vowel "wheel" a couple centuries ago - high vowels dropped lower, like a towards e, e towards i, and the lower ones got some upward movement towards a, like u and i.