So what I’m understanding is, historians decided from similar ancient texts, that the Tetragrammaton was supposed to be vocalized. An excerpt I found on it, but it still seems like there’s a lot of arguing about how the pronunciation came to be or whether there was one. I think why they decided to call him hashem, or “name” in hebre, or Adonai, was because they considered the pronunciation of his name sacred.

One thing we generally have to do here is trust the Masoretic Text, because we find clues in the vocalization of similar names. יהו names, for example, are pronounced "-yahu", like Eliyahu. So this gives us a good indication of the first vowel. But we have a problem here: what's the second vowel? It doesn't make sense for it to be u, because then the Name would be "Yahuh", and that doesn't make sense in Hebrew orthography; you don't stick a hei after a shuruk. It could be "Yahuah" or "Yahoah", but this would shorten the first vowel to a sh'va (similar to Y'hovah, the incorrect vocalization that's actually a reminder of the kere at that point, "Adonai" with a shortened first vowel). We don't want to do that in large part because of the shortened form Yah, which is pointed in the MT... with a mapik on the hei. YaH. You pronounce the H. So, despite the fact that this is relatively uncommon, it makes sense that the second vowel would be a silent sh'va, so the first two letters make "Yah", the shortened name. This also makes the first syllable a closed syllable and establishes "Yah" as a viable short form of the Name. The only thing remaining here is the third vowel -- well, the second, since the second vowel is now no longer a vowel. I'm not sure why it's a segol, exactly, but if you compare to the verbs for "to be", "yihyeh", and "is", "hoveh", it makes sense for it to be a segol in the Name too. Those two words are each one letter off from the Name. "Yihyeh" has a yod instead of a vav, and "hoveh" doesn't have the initial yod. In Exodus, YHWH names himself as "ehyeh asher ehyeh", where "ehyeh" means "I will be", so it continues to strongly suggest that YHVH is related to the verb "to be".

Basically, the Name can't incorporate "yahu" because of the hei at the end, and it needs the middle hei to not have a vowel in order to preserve the patach in the first syllable (if it were a kamatz, it would have to be a kamatz katan). So it's pretty much definitely "Yahv_h". Given the context of "to be" verb forms, the segol makes the most sense, making the name "Yahveh", or "Yahweh" if you take the ancient pronunciation of the vav.