Is there a recommended external capture device? I’d like something that can handle passthrough on an RGB modded NES, SNES and N64 if possible.
I don’t think there are any external solutions that correctly handle or support 240p in general or the jittery sync from the NES or SNES.
I expect you’re going to need to apply at least line2x for 240p. Most should support 480i just fine, but I wouldn’t be surprised if there are models out there that require 480p minimum (you’ll need to check product listings).
For the sync jitter, you’re going to need some form of dejitter mod. On the NES end, if you have a NESRGB 2.x (or newer, I imagine; 2.0 is the latest at time of writing), dejitter support is built in; if you have an older board, you should be able to swap in the newer model without much fuss. For the SNES, there is now a combination RGB bypass and dejitter board.