The Exeter Book was used as a cutting board

No. There was an obsolete letter þ. It is sometimes rendered as Y. It is totally unrelated to the letter Y.

There is also such a thing as ð, which I think is used in nordic languages? This looks like a d but is a different sound. In phonetics this is used to connote the voiced version of "th", like the th in "those" (as opposed to the "th" you would say in "the" which does not use the vocal cords). According to Wikipedia, the obsolete English letter þ was sometimes rendered as [ð], sometimes as [θ], which sounds about the status quo for "th" in modern English.

> ge had the sound of "yay" sometimes.

Ignoring the confusion of modern English vowels (note that pronunciation of foreign words with "e" often get transcribed informally by English speakers as "ay", eg. ¡olé! one might say is oh-LAY - that's half of what OP is doing here) ... It's not surprising.

If you look at the history of European languages, what happens to a G followed by i or e can get ... Flexible. A lot of Western Europe today decides that's like an English J sound. A lot of Eastern Europe retains a hard G. It wouldn't be shocking if older forms of English had that as similar to modern English y. (By the way, the original pronunciation of the letter J was meant to capture the times I was a consonant... I.e. more of a modern English Y sound)