The phrase "astonishing since" is correct and usable in written English.
It can be used to introduce a surprising fact or situation that has been ongoing or has a specific starting point in time.
Example: "The results of the experiment were astonishing since they contradicted all previous theories."
Alternatives: "surprising given that" or "remarkable considering that".
Smith's poor play is all the more astonishing since it comes after he finished the regular season in peak form.
He is unutterably charming, which shouldn't be all that astonishing, since charm is what film stars do for a living.
This is quite astonishing, since it's appealing to the sort of moron who also tends to think that Arabs are raghead terrorists who need bombing into submission.
In many ways, Arendt's approach is itself quite astonishing, since she is, among other things, trying to defend the relation between Jews and German philosophy against those who would find in German culture and thought the seeds of national socialism.
"You can say it's influenced conservative politics, but what's been astonishing since the financial collapse of 2008 is how the political establishment has held together, not how it's broken apart".
In any year, that would be encouraging, but for the Italy of 2013 it was astonishing, since the country was immersed – and remains immersed – in its longest recession since unification more than 150 years ago.
That batch of recent releases amounts to an astonishing six since the beginning of summer.
That is an astonishing shift since his childhood days of overwhelming monarchism; no wonder he is nostalgic.
It has been compounded by another astonishing transformation: since the 1950s, global life expectancy has risen from 48 to 68.
The German won most of her big titles when young; Williams, who will be 35 on 26 September, has an astonishing record since she turned 30.
It has attracted a modest 2,032 visits over the past hour, but an astonishing 4,014,535 since it was first put up two weeks ago.
