Convert to 8-bit PNG w/alpha (you don’t need more than 256 colors for this) then zopfli:
C:\Users\Jeff\Desktop>zopflipng Untitled.png Untitled.png
Optimizing Untitled.png
Filter strategy zero: 6459 bytes
Result is smaller
Input size: 10100 (9K)
Result size: 6459 (6K). Percentage of original: 63.950%
Gets it down to 6,459 bytes.