How can I resize a layer (or text) to a percentage of the canvas?

There might be a creative way with an action, but a script is what first comes to mind. Scripts can be recorded into an action as a step.

Try the following for just the document width:

var originalRulerUnits = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;

docWidthScaler();

app.activeDocument.resolution = originalRulerUnits;

function docWidthScaler() {
    var theLayer = app.activeDocument.activeLayer;
    app.activeDocument.activeLayer = theLayer;
    var scale = Math.min(app.activeDocument.width / (theLayer.bounds[2] - theLayer.bounds[0]),
        app.activeDocument.width / (theLayer.bounds[3] - theLayer.bounds[1]));
    // Scale to N% of canvas
    theLayer.resize(scale * 50, scale * 50);
    // Centre on canvas
    theLayer.translate(app.activeDocument.width / 2 - (theLayer.bounds[0] + theLayer.bounds[2]) / 2,
        app.activeDocument.height / 2 - (theLayer.bounds[1] + theLayer.bounds[3]) / 2);
}

Or conditionally based on Portrait or Landscape document orientation:

var originalRulerUnits = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;

// Conditional resize layer by portrait or landscape orientation
if (app.activeDocument.height > app.activeDocument.width) {
    scaleP();
} else {
    scaleL();
}

app.activeDocument.resolution = originalRulerUnits;

function scaleP() {
    var theLayer = app.activeDocument.activeLayer;
    app.activeDocument.activeLayer = theLayer;
    var scale = Math.min(app.activeDocument.width / (theLayer.bounds[2] - theLayer.bounds[0]),
        app.activeDocument.width / (theLayer.bounds[3] - theLayer.bounds[1]));
    // Scale to N% of canvas
    theLayer.resize(scale * 50, scale * 50);
    // Centre on canvas
    theLayer.translate(app.activeDocument.width / 2 - (theLayer.bounds[0] + theLayer.bounds[2]) / 2,
        app.activeDocument.height / 2 - (theLayer.bounds[1] + theLayer.bounds[3]) / 2);
}

function scaleL() {
    var theLayer = app.activeDocument.activeLayer;
    app.activeDocument.activeLayer = theLayer;
    var scale = Math.min(app.activeDocument.width / (theLayer.bounds[2] - theLayer.bounds[0]),
        app.activeDocument.height / (theLayer.bounds[3] - theLayer.bounds[1]));
    // Scale to N% of canvas
    theLayer.resize(scale * 50, scale * 50);
    // Centre the text layer on canvas
    theLayer.translate(app.activeDocument.width / 2 - (theLayer.bounds[0] + theLayer.bounds[2]) / 2,
        app.activeDocument.height / 2 - (theLayer.bounds[1] + theLayer.bounds[3]) / 2);
}

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html