I thought I'd share some of my PCB designs!
I've designed a new RGB circuit for all PC-E variants (including Duo's) that's based around the THS7374. It differs from the THS7314 by being a 4 channel driver, in addition to being able to turn "off" the LPF that's hardwired in the THS7314. Disabling the low-pass filter yields a considerably sharper image. I use the 4th channel of THS7374 to properly drive C-Sync output, so no additional mod is needed. I have both a Duo and PC-Engine/Core-Grafx footprint designed and tested. The PC-E/CG variant solders directly to the expansion bus pinout for simplicity. This board also attenuates the output signals to .7vPP for 75 ohm loads. Meaning, the image/colors aren't washed out or are overly bright.
Also, for the OG PC-E, I've built a "proper" audio-driver for the stereo outputs found on the expansion bus. These inputs are line-level off of the pins and aren't driven. So this little board will really "wake-up" that 'ole white PC-E.
I've also designed a more competent N64RGB mod for the NS1 mainboard revisions. This board is also designed around the THS7374, which also has the sharper image output with the LPF disabled. There's an LM1881 onboard that's properly terminated to provide TTL C-Sync for N64 mainboard revisions (CPU-04) that doesn't provide a "simple" C-Sync output.
Nevermind the dodgy soldering, this is just a quick test jig that I whipped up to probe.
There's also a variant of this for the SNES 1CHIP & Mini that properly attenuates the video output to the correct level, with all of those goodies on there, too.