Bresson was definitely obsessed with the notion of automatism and the fact that so many of our actions are automatic and done without thinking. (I love that he insisted on an "untrained" donkey, just as he did with his nonprofessional models.) But I also think he saw humans as extraordinarily complex, which is why he hated simplifying them into psychological types. But he definitely saw humans and animals in similar ways and was always interested in their relationship. Every time I see the end of L'Argent, (this is not a spoiler) with the dog running up and down the stairs, I think of Balthazar.Bresson sees humans as we see animals, provided we don't see animals as Disney documentarians do...
I don't know if I personally find Balthazar to be his most "spiritual" film, but I do consider it deeply so, in large part because of the film's intense gaze upon suffering and tenderness expressed with ellipses and mystery. The viewer wants to know why these things happen and is encouraged to speculate and imagine and look within. "Hide the ideas, but so that people find them. The most important will be the most hidden." (Bresson, Notes on the Cinematographer)
And I would separate Bresson's materialist style from a materialist worldview. I think there is much in L'Argent that would suggest otherwise, and even in interviews Bresson gave during that period, he spoke seriously about his belief in "another world." But Bresson felt that world through the precise physical sounds and textures surrounding him; it's quite a paradox, but I think his films beautifully convey it.