So I bought the Loopy controller board from ezladair on eBay.
It's not expensive but he did not give good instructions.
1. I got the purple board, the pictures use a green board, which is similar enough but needs missing steps.
2. The purple board needs the 2 horizontal exposed copper to be bridged and shorted to 3.3V. I only learned when I noticed the Wii diagram was wrong and asked him.
3. His Wii controller instructions are totally wrong! Like 3.3V and Ground are inverted. Thankfully he didn't inform in the pictures about #2.
4. There is an alternate trace for R for new 3ds XL and it's safer to do. I did the recommended and it broke R on my 3DS because the trace was really thin there.
5. The home button trace is so small. I'd skip it and just remap home to start+select. I broke mine and lost the literal almost microscopic diode
6. I'm glad someone pointed me to install the board on the lower left area and use wires to connect the 6 pins to the female connector
7. I had to cut the 6 pins since they were too long
8. You need to put an alligator clip on the 6 male pins for the controllers since they move around when heated.
It works now (despite R and home button broken) and having capture card is great but I don't know. I'm not super happy with it since it doesn't support all the buttons. Only the older 3DS have support. I might go crazy and buy an older 3DS XL + capture card and doing it again.
Also if you're going to hack your 3DS, do it before. I had hacked mine but I lost the original files, so restoring FBI was a pain with a broken home button (power button was a workaround) and broken R button. When connecting the controller in GodMode, it was scrolling down like crazy even though it's okay on 3DS and DS mode.