As far as I’ve found, there’s no good DIY way unless you’re a lot better with electronics than I am (which admittedly isn’t too great a feat… I loosely understand how the Cadet modules work, though I could not have designed one myself from scratch).
The way color is encoded in the composite signal is by encoding the chroma (color) information at a specific frequency and then mixing that into the luma (brightness) signal. So you’ll need to filter out the chroma, which shouldn’t be too hard (if you know how to design a specific filter), but in order for the chroma to be correct it needs to sync to the colorburst. So I believe you’ll need a PLL to sync to the colorburst, but I’m not entirely sure where to go from there (or even how exactly you’d sync the PLL). I found this link http://codeandlife.com/2012/10/09/composite-video-decoding-theory-and-practice/ which seems a good explainer of the problem at hand. I haven’t search much more, but the general concept is “composite video decoding.” It should definitely be possible, and possibly not hugely complex, but it’s definitely beyond me, for now.
There used to be handy IC’s that would do the composite decoding for you, and with those a Cadet-esque composite-to-RGB module would be fairly easy, but since “no one” cares about composite video anymore, they’re no longer made. One I found that seemed particularly suited to this is the TDA3330. It was primarily made for decoding composite video that’s fed into a CRT tv in order to drive the picture tube, but I found an application note that was for non-picture tube purposes that seemed promising (though I think the output levels still need to be attenuated quite a bit to get to LZX levels). I ordered a couple of second hand chips from somewhere just a week ago but haven’t gotten around to trying to use them yet.
You can find composite decoding ICs that are in production now, but they’re designed for modern devices so they’re usually very fine pitch and not very DIY friendly (like QFN, etc), and on top of that they decode to bitstream (for use in some sort of digital context) instead of to analog RGB.