I agree there. Despite being a horrible person off screen, Ezra Miller can actually act with good material, We Need to Talk About Kevin, and The Perks of Being a Wallflower are proof of that.
I think as you alluded to, the bigger problem was that Snyder’s take on Barry Allen was ill-conceived from the beginning. Ezra may have been a good fit for Snyder’s vision for the character but his vision was Barry Allen in name only.
I find it very amusing that the CW managed to do the character so much better for nine years and with a fraction of the resources.