Hello NeverDie,
nice that you're interested in my project. I've also planned some other MySensors devices like an easy-to-use Raspberry Pi Gateway or battery-powered temperature and windows sensors.

To your questions:

I have written instructions on how to solder the adapter cables for ISP and Serial programming. They can be found on my website (https://thdev.org/?Projects___misc___micro_JST). There is a link in the building instructions, too.

I've purchased the 8pin connectors and cables from AliExpress. Here are the links to the offers. I have bad experience putting these in the BOM since AliExpress offers often expire quickly but these are still online.
https://www.aliexpress.com/item/10PCS-1-25mm-Connector-Horizontal-Type-Straight-Pin-Female-JST-Socket-2-3-4-5-6/32787273682.html
https://www.aliexpress.com/item/10-Set-1-25mm-Pitch-Male-Connector-Wire-15CM-Long-28AWG-2-3-4-5-6/32787305004.html
If these should expire, just search for "jst 1.25", "smd jst 1.25mm" or similar.

Hope that I could answer your questions
ThetaDev