What did 2nd Temple Judaism Believe……REALLY?

[Koichos]

Ok so I’ll put it like this, is the “ancient of days” or son of man the “messiah” Jewish currently are waiting for to come back and rule?

Not quite. The Aramaic phrases עתיק יומין (Daniyyel 7:9) and עתיק יומיא (Daniyyel 7:13) refer only to Gοd and mean 'the One from the ancient years', that is to say, 'the same Gοd who existed in ancient times'. Conversely, כבר אנש (Daniyyel 7:13), which means 'something like a human being' but literally 'one like the son of a mortal' (not a 'gοd', or even a 'gοd’s son', but a completely mortal being), refers to the King-Mashiyaħ. And Daniyyel was only dreaming, as the text reads:

חזה הוית בחזוי ליליא, וארו! עם ענני שמיא כבר אנש אתה הוא; ועד עתיק יומיא מטה, וקדמוהי הקרבוהי׃​

I was dreaming [lit., 'I was seeing nocturnal visions'], and wow! [something] like a human being was coming with the clouds of the sky; he approached the One from the ancient years [lit., 'days'] and was brought before Him. (Daniyyel 7:13; see also 7:7)

Isn’t it only ONE individual who will be a messiah strictly from the Davidic line?

'Messiah' is just the title of our ancient kings. All our messiahs (with the single exception of King Sha'ul) have been from the Royal Line of Y'hudah. When the future, promised king rises to power, the ancient coronation ceremony will be reinstated so that he will be a 'messiah' in the most literal sense of the word, and for this reason he came to be known, throughout ancient Jewish culture and literature, as המלך המשיח hamellech hamashiyaħ or simply המשיח hamashiyaħ.

But the promised individual whom we call in Hebrew המשיח hamashiyaħ ('the Mashiyaħ') or המלך המשיח hamellech hamashiyaħ ('the King-Mashiyaħ') is never actually referred to as such in the Tana"ch: he is called by several different titles (מלך mellech 'king', נגיד naggid 'ruler', נשיא nassi 'prince'), but never 'a' messiah (משיח mashiyaħ), much less 'the' messiah (המשיח hamashiyaħ) - the latter of which does not occur in the twenty-four books of the Hebrew Scriptures.

Here’s a good video breaking it down using scriptures from a Jewish perspective

That's not a Jewish perspective.

All I know is, the more I started learning about Islam, it took my understanding of Christianity and religion to another level.

And if I ever learn HEBREW, watch out

You should! I myself have been learning with a cousin of mine the Judaeo-Arabic translation of Torah alongside each week's Hebrew portion (parashah).