V3 will not have a USB C connector.
Maybe when I get to V4 
There isn’t a lot of documentation, but what there is can be found here:
https://fredrik.hubbe.net/lightsaber/v6/test_rig.html
(Make sure to follow the links for the breakout board and pogo pin adapters for more info.)
Hobby-level stencils are not expensive, just go to oshstencils.com and order some.
For a V3 board, getting the boards is probably a lot more expensive than the stencils, because oshpark boards will not work for V3 proffies.
It can be tricky to get enough current through the pogo pins…
It’s a lot of work, but if you are having fun doing it, then go ahead.
The RAM has no impact, because the code doesn’t actually use the extra RAM for anything.
The extra FLASH can be helpful, because you can increase the optimization level when compiling the code.
The extra RAM does not help with audio underflows.
However, the V3 has an SDIO interfaces which can read data from the SD card faster, which does help with audio underflows.
I assume you mean turning on optimization?
It really depends a lot on what your styles look like, and how many LEDs you have.
If you have simple styles, it makes no detectable difference. If you have big complicated styles and lots of LEDs, it can make things much (maybe up to 2x) faster.
There are definitely some gestures and effects that care (like drag / clash / stab)