Is R59 missing? Hard so see dark components on that picture. And the wifi module in that area looks been reworked on the joints?

I remember, by default SSH is disabled in retropie.
So, maybe you need to enable this before be able to transfer files using WIFI.

My message (PM) went to outbox. I never understood if this delivered now or not!

Maybe something to do with the already known "joycon drift"?

Just have a look in the internet about it... Ok drifting and moving to another direction are not exactly the same buy maybe this helps your problem solving.

Beautifull results! I like if people use their knowlegte to perform what they like to do. This times it went really nice.

Somehow: please tell me how I can buy your designed product

Do we really want to start a discussion about solder with or without lead in it? Lead was always better for solid connections. You can be happy to still can use it for private! Many malfunctions on commercial devices are just caused by lead free solder. You just should not eat it and maybe wash your...

This part breakes on some shipsets. Some for me: just try to use the rest of the remaing plastic bar to lock your cable. Should work fine, this is just a mwchanical support.

I do not see any battery on your picture! There is the pcb with a hole in the middle and a potted piece which holds just your game rom inside it. There are solder pads for a small 10uF capacitor but was never soldered to it. I do not know what a capacitor is here disignated for. Storing game data wo...

Try ebay. Maybe in different countries. I found often so.e out there.

You decision is fully fine! No brand is nesessary to prefer, just maybe silicone wire since it is much more easy to solder for beginners. I suggest all signal wires to be twisted to its pair: USB, Audio and Video. Button wires and power do not nead to have this twisting.