I'd say the main things that could use improvement in your design are spacing, size, and affordance. This is not exactly a UX question, but more of a visual design question, but improving on one often means improving the other.
The space between items is an important and often overlooked part of design, and becomes an especially interesting balancing act with the ascenders and descenders of the type. With a design as simple as this, you could consider having just one or two spacing units. For instance,
- Can the spacing above the button be the same as the spacing below the button?
- You can create visual pauses by doubling the spacing in one or two places.
- It's also OK to not use all the available space in your layout.
Once your spacing has a rhythm to it, this will likely effect your size. You wanna make sure that each item has a proper weight to it so that the items are read in the correct order. Also, if you think your text might be too light or too hard to read, it probably is. It's almost always good to err on the side of legibility for our friends with poorer eyesight.
I think with a design this simple, your buttons are probably sufficient. But there are a lot ways to make a button seem clickable:
- Hover state (for desktop design only)
- Rounded corners (you've already got this)
- Depth (dropshadow or embossing)