Limit the palette
Two colors plus a dark and a light. The tool will offer more. Ignore it. Consistency reads as quality.
Make the cover readable at a glance
Title, subtitle if needed, no paragraph on the box. If you cannot read it as a tiny thumbnail, it will fail in an email.
Stop at good enough
This is launch graphics, not a portfolio piece. Export, put it on the page, write the letter. The download is there so you are not stuck waiting on a designer for a badge.