Thanks !
been able to implement it like this:
const ease = new BABYLON.BezierCurveEase(.65,.39,.26,.54);
function zLerp(start, end, value) {
var tMod = ease.easeInCore(value);
var result = start + (end - start) * tMod;
return result;
}
Thanks @Evgeni_Popov !
(numbers taken from https://cubic-bezier.com/)
see it at work: