From your rationalisation I would think a set of Diamine inks would be within reason. Ink is a bit expensive in general, but you get 10 x 30ml bottles in a box, at least in the flower and music line. You might have to add a color as an additional purchase, but it's not a bad workaround. For the odd decorative touch up, a dip nib and small ink bottles aren't bad at all either.
This is one of the set I am tempted to buy too. Herbin have as mentioned nice ink sets too, but again, you probably need to add a color or two to complete it for your needs.
I have to say, I love writing with deep colors on a nice cream paper, blue-black, deep burgundy hues, dark greeen, dragon red, slightly flexy nib,... I understand your preferance for a nice black with any type of writing too.
I just noticed there are mini Iroshizuku bottles, probably the least expensive way to test these inks.
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