I also don't like the supplied AV solution that the NESRGB kit has. At the time I purchased the kit (which is a few years ago now) I was using the Framemeister almost exclusively, so having that style mini-DIN made sense. But I don't like how it's a PCB mount jack, and you're suppose to just glue it in place. So I had purchased a proper panel mount mini-DIN for it...
I still wasn't a huge fan of that AV cable you linked to. But I get it. IIRC, it breaks out the audio because that'll take out any crosstalk/interference. My plan was to just wire the mini-DIN the same as the Framemeister, then just use a straight through cable for audio and video. But I ended up going back to CRTs and stopped using the Framemeister. So I ended up picking up a 3D printed Nintendo AV multi-out port, so I could have all the AV signals and use standard cables. That Voultar FFC is cool, I haven't seen that before (unless, like you, I also forgot about it). I don't have that, but I do have Voultar's (or someone else's) PCB that mounts the needed resistors on the front loader's expansion port to get the expansion audio.
Yes, desoldering that PPU is a huge pain in the ass with out the right tools! Mostly because they are plated through holes, making desoldering very hard especially on ground plane pins. When I purchased my NESRGB kit, I didn't have that great of equipment. Now I have a Hakko FR301 and a hot air station (I find it easier to suck out what you can with the vacuum, then hot air the chip out instead of reworking stuck pins). But I just never got around to installing it.
But dammit, Anapan, you're giving me the bug. Not only this post, but the RGB modded AV Famicom I got from you. I've actually never seen RGB from a NES in person on a CRT before (it's not the same seeing screenshots). When I tested out the AV Famicom, I was wowed by how good it looked. But I'm undecided what I want to do. I have an RF Famicom that I may want to take the PPU out of and put in the AV Famicom that I got from you. And I think I want to get another NESRGB, the new V4. And between my top loader NES and AV Famicom, put the old and new NESRGB kits in those consoles (undecided which gets which). Then with the PlayChoice PPU and the RF Famicom, I've long wanted to make a sort of steampunk Famicom and it would be nice to have more than the old composite video mod for it.
Anyways, nice work! Is that the extra board for component video I see on there?