something caused the fluctuation, etc etc, and eventually you have to have a first cause.
Not if fluctuations happen by their very nature...
I don' t know enough about physics to argue whether the big bang was caused by quantum fluctuations. I just don't see any reason to believe that rest is the 'default' state and motion must be caused. I don't even see that there's a case to be made for 'nothing' to be the default state instead of 'something'. There's simply no empirical evidence to back up these sorts of claims.
If we can define a time which is the start of the big bang, then time exists outside of it,
No - you can't. It simply isn't there. Before the big bang there is no time. There is no space. There is no geometry. There is nothing. Not 'vacuum' nothing - nothing nothing. Not even space for there to be something.
Talking about a time 'before' the big bang or 'outside' the big bang is incoherent.
As such, if we define time as a dimension just the same, then we can define a position in the range of negative infinity to positive infinity. The big bang is just a point on that line.
No - you're fundamentally misunderstanding the big bang. The big bang is t=0. There is no 'before' the big bang.
If you want to say it created time and space, then it's more like saying it created the values of time and space we pass through, not the dimensions themselves
No - it created the dimensions themselves. There was no place and no time for these dimensions to exist until the big bang took place.
Google time and space before the big bang and you'll get a lot of articles on this very subject
Furthermore, cause/effect does not necessarily require the dimension of time, even though that's how we typically think about it.
Again - absolutely wrong. Cause and effect are temporal. Cause precedes effect except in unique circumstances such as the big bang where they happen contemporaneously.
You've got to get the notion of time and space out of your head prior to the big bang. There is no 'prior' to the big bang. It's very difficult to even wrap your head around the concept of what 'nothing' implies. No geometry... what is that? No time.. what is that? That's the fundamentals of big bang cosmology. Prior to t=0 there was 'nothing'