Par Gerell's serve

Dimitrij Ovtcharov's serve

Tomahawk serve

Pendulum Serve except you brush under the bull making half a 'u' shape (almost like a curved 'L').

Reverse Pendulum

Vladimir Samsonov's serve

I think I disagree, but it all depends on how often you play tournaments, what level you are at and how your game evolved.  Serving practice rewards all the other parts of your game, especially when you get someone to teach you how to practice serves correctly.  The mistake most people make is that they have never received formal serving instruction so they don't know how to work on their serves.

The most important thing is to have a couple of serves that are not easy to attack so that you don't always feel under pressure when playing aggressive opponents.  Usually, that means at the very least being able to serve low and short.  That by itself requires practice for most people and also assumes that you have good receive, because if you don't, not getting anything out of your serves will put you under pressure.

When you play 2100+ players, the biggest problem is usually how to get the chance to play your game.  If you don't have a serve they cannot attack, you will not have any fun.  If you do, you will at least get to the rally sometimes.

You are also a lefty so your serves are unusual just for that reason.  That makes it a bit easier for you, but trust me, you are probably leaving a lot on the table by not focusing more on the serves given your innate advantage.

A friend of mine who is a very good player (top 50) in Belgium has only 1 serve, a very very short ball, without spin just in the middle of the table, just behind the net. This ball is impossible to attack and that's all my friend needs to develop his game. The only thing he dislikes is being attacked directly on his serves. No need of spin, speed, variations....

An other friend has more than 15 different serves, developed for years and many many hours of training. He usually began at his home, using his wrist, paddle and balls (no table) to experiment the variety of spin he can generate with different moves, then once he believe something deserves a experiment, he tries it at the club.

Videos are interesting in slow motion to watch the moves of the body, the arm, the wrist but it will never replace hours of practice...That's the price to pay to get easily some free points :))

If you're already doing a pendulum serve with either pure side, side top or side bottom, then you really have all you need. Now just try to disguise it well and above all else, keep it low. If your bounce is too high, it doesn't matter what spin is on it. That ball can easily be attacked.

If you really wanted to add to the variety of your serves, you could also add...

- A no spin serve. An easy serve to do that you really should have. It's just a simple serve with just a hint of backspin. After the two bounces (which add topspin) that take place before they can contact the ball, it'll be no spin. Have a short and long fast version.

- Pure backspin. I assume you already have this. It's heavier than your pendulum side/back serve.

After you put those in, if you really want to, you could add a reverse pendulum serve. This one is harder to do for some people. The good news is that you have an easy out if you can't do the wrist motion. Just do a punch serve. It creates the same spin and is easier to execute. (see video)

I think the important part if you want to improve your serve is to find a good serve that works with your game type.

My suggestion is that you look on youtube on players that you find using your game type and trying to look at there services. Look carefully at the placement on the table and where they usually gets the return.