So, I tried training by tying a thread above the net. I got the hang of it after about half an hour or so. I would say just reducing the height has elevated my serve by about two levels. I got a lot more direct service points during my matches with my regular partners. But I am also dumping a couple of serves in the net right now, and a LOT of let serves.
Reducing the height though has one major side-effect. The topspin serves travels even more horizontally, and it has become a bit easier to read. People take it late and are able to keep the returns quite low as well. Same with backspin, it is easier to push short and keep it low, effectively taking some bite out of my 3rd balls. I am getting a lot more returns that are not as attackable and am forced to push.
I guess I have to upgrade my 3rd ball as well, or move the attack to 5th.
Serve is a very high level task and one of the most difficult skills to master.
I had to train 1 hours a day, 7 days a week for few years to get to my level and I am still very low compared to much stronger players.
it isn't only about height, but rather to control the power on the ball. How to generate a very strong spin shot, but allowing the ball to be light. The placement is also so important.
Or to add in fake action to trick your opponent on what you are serving.
There is the contact point trick and then there is follow through trick.
keeping the ball low means you won't be killed 1st ball by stronger opponents.
But ultimately, keeping the ball low is really just one of many things you need to do well.