Basically, I had a green CF-zif adapter board and I used a red CF-microSD adapter with it (and a sandisk 256 GB microSD). I'm aware that the iPod classic 6 only supports 128 GB stock, but I haven't heard reports of it not working when you put in more. It worked perfectly fine on my iPod 5th gen (5.5). I believe that I had to use AOMEI to rebuild the MBR, or Victoria to rewrite the first byte as 00, but other than that, it worked.
Unfortunately, I dropped that iPod and it no longer works (sparks, starts smoking, and overall creates a fire hazard when plugging in a battery). When it fell, the iPod fell apart and I think the green board might have potentially faced some damage, although I wouldn't really expect that to be the case.
However, when I attempt to restore the iPod classic 6th gen with the green board and red CF adapter, I get "error 1434" on my Mac (I even tried with Catalina and High Sierra). On Windows, it seems to work, but it just reboots right back into the "Use iTunes to restore" screen and iTunes pops back up asking to restore. This is the case even when using AOMEI to rebuild the MBR. Also, the Victoria method of replacing the first byte, which is "EB," with "00" that is suggested on r/iPod is a little confusing, since the first byte is not "EB" when I plug the SD card into my computer (though the second byte is.) Rewriting either of the bytes or both or neither makes no difference.
Putting the iPod into disk mode with the SD card shows absolutely nothing when trying to open it in Victoria; it only shows zeroes.
I also tried installing rockbox manually, but the rockbox bootloader says "no partition found."
TL;DR is the issue realistically just with the green board, or is this method just not compatible with the 6th gen for strange reasons? I know that the red adapter is known to be problematic, but the workarounds for it (which worked on my 5th gen) don't seem to fix it, and I'm pretty sure the SD card is fine.