Today, I slightly tweaked the grayscale to make it a little less blue-ish. I also decided to go with the dark grey for the main button, as Mario suggested.
I think the blue might convert better because users are more familiar with it, but keeping it monochromatic adds more character to the brand (somewhat ironically).
It might be worth doing an A/B test here. I'm generally against A/B tests because they tend to get overused by insecure designers, but there are times when it makes sense (usually when money is directly involved).
Next up
Apply the grayscale to all the components in Figma. Boring, I know.