Making an efficient constant current source
I suspect you're overthinking your problem. Maybe not, but probably.

There are two questions you need to ask:

1) How stable is your 24 volt supply?

2) How precisely do you need to control your LED current? 

You specify 21.04 volts and 55 mA for your LED string. First off, using 4 significant figures for LED voltage shows that you don't really understand the variability of LEDs. Second, since you are looking at using these LEDs for "illumination and decorative purposes", I suggest that controlling current to more than 10% is a big waste of time. You simply won't be able to see smaller differences. For some reason you seem to have settled on a forward voltage of 2.63 volts for your LEDs. This will change with, among other things, LED temperature, and you are assuming a stability which simply won't occur.

So, what I'd do is the simplest possible circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

The resistor should be a 1/4 watt unit, with some airflow to cool it. Don't pack all 31 units into a tiny sealed box. If you do, since they dissipate a total of about 5 watts, they'll get hot and either fail or burn something.

You will encounter statements about how switching regulators are preferred for LED control due to their better efficiency. That's true in general, but it's not much of an issue here. A simple resistor in this case will have an efficiency of about 87% (21/24) and that is right up there with switchers. Plus, this circuit is the ultimate in simplicity and low cost, both of which I suspect are good things for a first project. So don't sweat it.

Look - you plan on doing this, right? So you'll need to buy the LEDs no matter what controller you use. Buy the LEDs and a few resistors, then make a preliminary, simple version. Try it. If that is satisfactory, there is no need to get more complicated.