Lewansoul Libraries v0.1
https://penzu.com/p/923e5dc9

Hello
I have been busy with work those days, sorry for not being active.

To summarize, I have been working on 3 topics:
1) Try to reverse engineer the board
2) write acceptable code to drive the servo (library)
3) An issue while reading values from the servo (same than you? maybe).

Considering the board:

It seems to use the MCU (microcontroller) to receive and send the answer, so the activation of en-RX and en-TX is done by software on the pic....
This make it idfficult, because writing a MCU just for that is an big task.

I am looking now to create a timer circuit with a 555, detecting the first "low" bit of the communication and setting en-TX for the time of transmission of one byte. I ordered the 555, but is not yet here (probably tomorrow or Wed).

Considering the Library:

All good so far, except that the reading does not work reliably: the signal looks good on the oscilloscope, but probably the timing is poor.
Doing loop-back of the RPI -> RPI works fine, but when the servo board enters in action, communication works randomly.

IMPORTANT SYNC:

Reading your comments, it seems that you have issues with the 3.3V - 5V conversion? If this is the case, I have solved this and (at least from oscilloscope) it works well.
Did you managed to read reliably from the servo? In my case I did not.

UPDATE: ok, the issue reading was due to the UART misconfigured on the RPI, now it work perfectly.

I used:
$raspi-config
5- Interfacing options
6- Serial
login shell to be accessible over serial? NO
serial port hardware? YES

After reboot all works fine. So I will continue the code until I get the 555 chip