AVR-Guidance VS tl59025-devboard

Check out the newer attiny chips......cheap, available, and LOW POWER. No external crystal or other components required. Programmable via Arduino IDE.,,see: https://github.com/SpenceKonde/megaTinyCore You can use any arduino as a programmer (JTAGTOUPDI) or make one with a cheap serial adapter: https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md

For Serial UPDI attach either a diode or resistor between RX and TX, then connect RX to UPDI PIN, along with and VCC and GND. https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md

I own a PICKIT4, so normally I would just breakout Vcc, Gnd, and UPDI pins and directly connect my PICKIT to it. This time though my friend recommended me to also include a SerialUPDI circuit on board to accommodate Arduino users (and ones without PICKIT in general). The part related to SerialUPDI are JP3, JP4, D35, and R5.

Yup, that should do the trick. You might want to give this guide a good read-through. It has some chip-specific tips (search for FTDI) and general recommendations for wiring and removing the Rx LED if it gives you troubles.

Choice of hardware

You can use RX/TX with the UPDI interface directly, see https://github.com/SpenceKonde/AVR-Guidance/blob/master/UPDI/jtag2updi.md. You should at least add a resistor on the line. Then you'll be able to program it with pymcuprog in serial mode, without a bootloader.

ISP looks a lot like spi. Updi is asynchronous uart. It's a lot different and I would recommend looking at serial updi.

The full schematic is here if needed, thanks: https://github.com/Sirawit7205/tl59025-devboard