Hi Andrew,
I love the Kawekos for their vintage art deco look and for the fantastic range of colors of their cartridges (I think they are my only fountain pens with which I don't use a converter and normal ink). They are small for my big hands, but I love them anyway. I used to write with broader fountain pens but lately I have became an adept of F or EF nibs.
I practice Spencerian with any tool I have around (pencil, ballpen, fountain pen...), but when it come to dip pen, I always use an oblique holder. It looks intimidating at first sight, but I can tell you it saved my life (speaking in calligraphic terms). I am right handed and if I want to write on the slant with a straight holder I have to turn the page 90º anticklockwise and I completely loose the control over the letterforms, spacing and so on. I feel it almost like writing upside down. Even with the oblique holder I have to turn the page quite a lot and, despite of it, I fell my nib is not well aligned (that is very patent when I try to make the shade of the capitals). Well, it always puzzles my why the old Spencerian instruction books recommend a straight holder (and a very uncommon grip), whereas they were aware of the oblique, as sometimes they recommend it for artistic penmanship (exactly the other way around to what we see nowadays, I think). But this is a question for another thread and for the specialist in this forum.
I love vintage nibs (I love vintage stationery in general!), but all the ones I have are under arrest until I learn to write with the Nikko G. By the way, they are meant to be indestructible, but I just spoiled a quite new one writing an address on a nice envelope of very textured hand made paper. Well, I have learned the lesson. I know about the J nibs and I have seen pictures. I think they were produced by different companies and were meant to be used for everyday clerk writing. I have quite a few books about fountain pens and there are loads of them in the market, but I don't know any book about dip pen nib history. However, I found this amazing webpage, which is surely know to the members of this forum:
https://thesteelpen.com/.
Keep practicing. Cheers.