Inky:
I think I understand your dilemma.
No ink is good enough for any job, not Canon not any.
For example, I print photos of coins with a slight tarnishing, and most inks dont recreate a realist color. On prints of gold and silver coins the slightest shift in color is magnified many times, so I must use a different ink if I want realistic colors when printing coins.
However, if I use the same ink that I use for coins, when printing portraits, the portraits are not rich enough and the faces look pail.
In some other case I need to use another ink.
For printing coins on the ip5000, I find that the Costco IMS ink, gives me the best results, and I think that is what you are looking for. Costco ink is also a little darker than others, giving better details.
Most people do not like ink that produces exact colors, for the pictures do not come out vivid enough. Even when you bring regular film to develop, the colors are not exact, they use richer colors so that the picture looks more alive.
Also try using all different settings from the printer driver, they give all kinds of results. Try darker, lighter, auto, none, vivid, optimized, Photo Paper Pro, Photo Paper Plus Glossy, etch.
The truth is that inkjet printers are not yet capable of reproducing screen colors exactly.
I also experiment in mixing different inks, and sometimes I get very good results. But beware when changing inks in your printer flush it first with cartridges filled with Windex and Alcohol, or you may get a bad clog.
Also the slightest clog in your printhead will give you a color shift.
I wish I could see one of the photos you have the most problems with.
Robert