How to interface UART servo? (LX-224HV)
After some time researching the topic together @tlfong01 and me(@adrian_maire), we managed to get the servo working through the Raspberry Pi (from now: RPI) without the need of the driver board.
For this purpose, several problems has been solved:

Converting voltage level from 3.3V (RPI) to 5V (servo bus)
Implementing the different UART messages for the RPI
Converting full-duplex UART to half-duplex UART

Converting voltage level from 3.3V to 5V
Even if this could be a sub-optimal solution, considering that the 74HC126D allows level conversion, two of them has been used to manage both problems: enabling and disabling of Tx/Rx and level conversion at the same time.



simulate this circuit &ndash; Schematic created using CircuitLab
Implementing the different UART messages for the RPI
A project has been created (GPL3) to any of you to use it.
https://github.com/Escain/HiwonderRPI
(The code is unfortunately too large to include here.)
Converting full-duplex UART to half-duplex UART
For the conversion of full-duplex to half-duplex UART, the servo relay on the enabling and disabling of the full-duplex TX (transmission) and RX (reception) line:

When RPI transmit, it enable TX and disable RX
When RPI don't transmit, it disable TX and enable RX so the servo can answer.

The servo start answering UART request some 0.1ms after reception, this is below the OS scheduling time of any modern operating system, including Raspian. In consequence the first attempt to drive en-TX and en-RX by software was totally unsuccessful.
To solve this issue, the en-TX is implemented by hardware, through a retriggering monostate 555 timer of around 0.1 ms.
The en-RX is just the negation of en-TX.



simulate this circuit
Connecting all together:

Tx is connected to the RPI GPIO 14
Rx is connected to the RPI GPIO 15
enTx from both circuits are connected together
Servo S is connected to the signal of the servo
Gnd and VCC (3.3V, 5V and 9V) are connected to the circuits and to the servo. (This servo is HV and thus, support up to 12V, but others Hiwonver servos can NOT manage 12V, be careful).