I think something like this should do about the same.

/// ===============================
/// LFO ENGINE
/// ===============================

const LFO_RATE = 0.3;
const LFO_DEPTH = 0.25;
const TIME_SCALE = 0.03;

reg phase = 0.0;

xypad.startTimer(30);

xypad.setTimerCallback(function()
{
	phase = (phase + LFO_RATE * motionSpeed * TIME_SCALE) % 1.0;

	const angle = phase * Math.PI * 2;

	lfoX = Math.sin(angle) * LFO_DEPTH * 0.5;
	lfoY = Math.cos(angle) * LFO_DEPTH * 0.5;

	updateXY();
});

any reason you used sin twice with extra logic rather than cos?