The FT-4 also uses a 3.3v serial connection for programming, but it's 2-wire, meaning the Tx and Rx are both on the same wire, named DATA. I use the NanoPi Duo's serial port pins (Tx and Rx) and GND.

The FT-4 presents GND on the 2.5mm MIC sleeve, and DATA on the 2.5mm MIC ring. I connected GND directly to Pi GND,Data directly to Pi RxD, and Pi via a 2K resistor to DATA. CHIRP successfully downloaded and uploaded.

So, we have two worked examples of modern HTs directly connecting to the 3.3V serial ports of modern tiny SBCs.