Ok, so I actually kept looking and I found this amazing custom keystroke script that worked great!
var spans = event.richValue;
if (spans !== undefined && event.willCommit) {
for ( var i = 0; i < spans.length; i++ ) {
spans[i].textSize = 10; // font size
spans[i].linespacing = 15; // linespacing
// restore line breaks
if (spans[i].endParagraph) spans[i].text += "\r";
// reset styles to default
spans[i].fontStyle = "normal";
spans[i].fontWeight = 400;
spans[i].strikethrough = false;
spans[i].underline = false;
spans[i].textColor = color.black;
spans[i].alignment = "left";
spans[i].fontFamily = ["Arial"];
}
event.richValue = spans;
}
Thanks also to Alex at https://stackoverflow.com/questions/25380882/linespacing-in-multiline-fields