I’ll never understand how a suffix like -eaux even exists, and essentially melts down to -oh.

It's actually worse. It's two suffixes. With one of them typically having no phonetic realisation.

The -eau suffix used to be pronounced [ɛ̯au̯] → [e̯au̯] in Middle French, that's why it's spelled this way. Then the [au̯] got simplified into [o], and the [e̯] got lost; but by then the orthography was already standardised, so it got stuck this way.

That isn't even the weirdest part of it. That -eau is cognate to Italian -ello. Nowadays they neither look nor sound even remotely the same.

Then there's that -x. It's a plural mark. Originally it wasn't even -x, it was a ligature for Latin -us that kind of resembled a ⟨x⟩. Languages hate this sort of "similar, but different" situation — if it looks like ⟨x⟩ it gets treated as ⟨x⟩, so the ending -s for what used to be /s/→/z/ got spelled as -x.

Then French got rid of all syllable-final /s/ and /z/; you see this for example in the name "Descartes" being pronounced as /de.kaʁt/, note how there's no /s/. That applied even to plural /z/, that became Ø...

...well, in most situations. See, plural -s in Romance languages often piggybacks the next word, if said word starts with a vowel, becoming the onset of its first syllable. But then it wasn't a syllable-final /z/ any more, so it avoided deletion. So that ⟨x⟩ is still haunting people around, both to highlight "hey, this is a plural!" and because sometimes it's actually pronounced.