My position on this is that the Maker exists.
But he is not like he is regularly believed as.
Yet perhaps this kind of belief in him may be tied to how he exists.
I'm getting a 'it exists because we have faith in it, so we make it happen' sort of vibe about the Maker and a lot of what feels like the 'blue lyrium' side of things.
I also think that there is more going on than the Maker, even as he may be the most singularly powerful (yet also abstractly understood) entity in the DA setting.
We may never know if he exists and in what exact way, but I think that's the nature of him.
Ultimately, I view him as a force. The details of that force is not yet known. The Andrastan Chantry itself may always be debated on whether it is a lie or truth, at the core of itself at least.
We may always take sides on whether it is just a type of magic at play when it comes to anything Andrastan/Maker, or that the magic itself is sourced from the Maker and/or his works. I guess I take a sort of middle ground, where I think that there was a Making force to Thedas+the Fade+etc, but it is more the belief in the Maker that comes to form the 'Maker' we 'know' today. That as people anthropomorphized a particularly powerful Spirit, it gained more and more understanding of us and became what we hoped/wished/feared he would be.
I find it hard to believe that the DA setting was fashioned by a cosmic man, but I can believe that a more abstract chaotic entity eventually brought order and logic (of a sorts) to the earliest known version of magic (aka the 'matter' of DA), eventually created spirits, and from that, made mortals (perhaps in the shape of the Maker+spirits). Feedback loop of dreams and magic and fears and hopes and thoughts etc etc, made the Maker more and more 'human'. But in this transformation, we also see the results through more and more spiteful (though sometimes empathetic) actions of him.
Basically, I think the Maker is not really the Maker, but we call him that so he kinda is.
So I guess the Maker doesn't exist. Or he kinda does. Or its just complicated, so I just go 'this series is about magic' about everything, in the end. Anyway.
The theme is hope, faith, that sort of thing. In this sort of way, the 'existence' of the Maker (existence in a way we understand 'physicality') does not really matter. Or it does, but more on our end - how tied to the mortal realm do you want to be? The more or less you are, the more or less you may regard the Maker as real. Or not, since many spirits are not concerned with Maker stuff in the least. But at least you'd be more open to him.
I do think there was a creative force to the DA setting. Whether to call it a Maker... may be appropriate enough for the current times (of Thedas), but may not have been the right way to describe it 1000s-10,000s of years ago. Recall the subjectivity about even just the elven 'gods', subjectivity that Inquisition often described and pointed out, perhaps is if they wanted us to understand how things really may work in Thedas, and even in Bioware stories and settings from here on (IMO).
This was a lot of rambling and contradictory opinions, but that's just how I tend to think about this topic. There is no definite answer, and there may never be.
Bioware may be hinting some things about the Maker when they employ mirror imagery too. And the latest introduction of time magic.