So I basically translate it as "For Subaru, those footsteps that echoed absurdly loud were, at the same time, the source of his impatience and also the one thing he could rely on."
What confuses me here is 「であると同時に」. It is the same as 「だと同時に」, right? Either way, I don't understand this construction/grammar.
I was thinking that maybe it would be ok to rephrase it like this:
Also, despite the grammar points, is my translation close enough? Those are the first lines of a book I'm reading btw.