At least from what I've seen, pros will use the long serve down the line 3 ways:
Using the regular pendulum serve:
- Gives them the ability to generate heavy spin to set up a counter like a heavy backspin serve lifted by the opponent and then attacked by the server.
- It can kick into the body and jam the receiver.
- However, the side spin curves into the body and can make a long serve less effective.
Using the reverse pendulum serve:
- It can kick away from the body and escape the receiver.
- Helps go down the line with the bat angle used to create spin.
- Has the ability to have spin and be stabilized.
- However, it can be hard to get extreme spin and/or speed so sometimes it just may not be enough.
Using the bat completely flat:
- It has deadly accuracy.
- It is insanely fast, often times they just don't have time to react or don't even try.
- However, due to the lack of spin on the ball, it may not always go on the table and it can be attacked if poorly executed.
The serves are situational and effective when used suitably rather than spontaneously.
From the video, it seems as though your long pendulum serve is very good, and you just need to keep serving it and eventually you will be able to get it on the corner time and time again. Hitting the flat, fast serve shouldn't be too hard and I assume you may already know it. Same action and everything, just hitting it flat and on the back of it, rather than brushing and hitting on the side. This also means it will go down the line.
The reverse one is probably the hardest out of all of these. Not exactly natural and harder to generate spin or pace. I don't have an exact video tutorial, but I feel like this short clip of Jan Ove Waldner serving the long reverse serve should give you an idea on it.
I'm not sure if it's not letting me go directly to the right time, so just skip to 3:55.
He mainly just changes his bat angle and swings through with a more flat contact. Honestly he makes it look easy, and I'm sure it isn't, but I think that should give you some idea. I've tried it out and it seems it's not a hard serve. It just takes some practice and effort with the right technique to develop consistency and accuracy on the serve. There's a little montage of Jan Ove Waldner so it may give you some ideas for other serves as well. He is also featured through out the whole video.