These days I've been using a standalone AVerMedia ExRecorder 330 for HDMI captures, that's how the ones in the blog were captured (via the 5X, of course). The older blog posts I link to I think I was still mostly using an Elgato but it didn't work at all on Mac, so I can't imagine their Linux support is much better.

The CXADC sounds interesting but unfortunately drops raw samples upon seeing input sync pulses unless you greatly decrease signal amplitude (which may be fine given it's 1-bit). I wonder if inverting the composite signal would bypass this. Alternatively the Domesday Duplicator is more expensive and may not be intended for composite signals. TBH I wonder if an ARM microcontroller could PLL to the sync pulses, sample the active signal digitally, and output a HDMI signal for a capture card (transmitting to a PC via the Pi Pico's 12 Mbit/s USB is marginal).

While researching LumaCode (similarly based on extracting digital signals from retro hardware) I came across RGBtoHDMI and https://lukazi.blogspot.com/2022/02/apple-ii-video-to-hdmi-u.... I have not read the article but it may be useful.

A problem here is that it doesn't have composite input like you'd need for the Apple II. So you'd need a Koryuu or the add-on I think they're selling now, which complicates things a bit. A solvable problem, of course, and maybe I should've included that in my post as well. Probably would have if it was a bit more serious!