The simplest and most reliable way is to divide it into multiple parts.
For example, split a layer of lines, a layer of skin paint, a layer of clothing paint, etc., keep each within 15kb, and combine them in the game to create a single illustration.
It is also important to reduce unnecessary nodes as much as possible when creating illustrations in order to reduce file size.
The number of nodes is directly related to file size. Minimizing the number of nodes while drawing the same lines will greatly reduce file size.
You can then use tools such as
SVGOMG to have the file size reduced even further.
There is a limit to the amount of space that can be reduced with this tool, and reducing the file size will also reduce accuracy, but for files up to about 30kb or so, it is quite possible to keep the file size under 15kb by using this tool.