In my opinion, if you're looking for a truly high end pen kit that looks and feels like a slimline, then you are out of luck. . There aren't any.
I agree with the comment that you have to get away from the Cross refill if you want any kind of "quality" in your products.
I also agree with some "alternatives" to the slimline pen kit but "alternatives" is really kinda misleading because they are totally different kits in just about every way possible. . They all use Parker-style refills which can be bought with quality performance (Schmidt brand refills and Private Reserve brand are two examples).
The Sierra Elegant Beauty, the Aero, and the Majestic Squire (you need a pry bar to open the clip if you want to clip it in your shirt pocket) are all appealing in their physical design. . They all use a single piece pen barrel, as opposed to the two piece barrel of the slimline. . These 3 pen kits have definite potential appeal to the ladies but maybe not so much to men ... who knows !
I cannot recommend the Tiny Giant because, although you can fit a quality Parker-style refill to it, it has a snap-on cap which you have to remove in order to write. The snap cap is not only inconvenient (it does not post to the rear of the pen when writing, as far as I know), but it is not aesthetically pleasing in form and taking it off and replacing it is a sort of rough operation required a bit of effort ... in my opinion.
I also do not recommend the Long Clicker mentioned above (Long Clicker is a CSUSA pen kit ... or Dayacom ... and has good quality) because it has a very clunky click mechanism ... you can feel the click very distinctly and you can hear it easily across the room. . I do not understand why the manufacturer does not improve this because quiet click mechanisms are not rocket science. . But that pen is an old design and modern advances do not get adopted very readily in pen science.
The closest I have come to a slimline format with quality potential is the Slimline Pro kit from PSI (others sell it, but I believe it is a PSI design). It uses Parker so is slightly fatter than the slimline (slimline brass tube = 7mm nominal, Slimline Pro tube = 8 mm nom.)
I know ladies like a slimmer pen and that is part of the slimline appeal, but don't forget that the slimline is a twist pen and that requires two hands for most people. It is not a great choice for everyday use by people like nurses who need a pen with one-handed operation.
The Slimline Pro is a click pen, so would be suitable for nurses and others with similar requirements. It is also a two barrel pen, but that is an incidental fact since the two barrels are not required to move relative to each other as in the slimline.
I like the Slimline Pro concept very much, mainly because it is slim-ish AND because it is a click. . But also because ....
I like the Slimline Pro best for another possibility it presents. Instead of making two barrels and joining them (permanently) with a coupler and center band, this kit can be made with just one, long barrel. . And that is neat, because it looks good ... shows off both the material you use and your workmanship in turning and polishing. . Plus it allows you to do innovative things with the long barrel. . I have been making Slimline Pro pens with a single barrel for a couple of years and am really thrilled with how they look and feel (long polished barrel). . You can even make them without any brass tubes at all. This is neat also, because you can get away with a smaller diameter hole through the pen blank, just big enough for the Parker refill to slide through, and the smaller diameter hole allows you to have thicker material, obviously. . I am referring to the difference between the inner diameter and the outer diameter of the barrel .. The thing that could be better about the Slimline Pro is that the metal components and platings could be much improved if the manufacturer would only consider these options. If that were done, the Slimline Pro could actually be a high-end slim-ish pen.
You can also make the slimline pen as a single barrel pen with no center band ... see an article or two in the IAP Library. However you are still stuck with the inferior Cross style refill.
Just for the record, there is an article in the IAP Library for making the Cigar pen in a single barrel version instead of the two barrel original design. . So this offers you the opportunity of making the Cigar overall slimmer than originally designed, and, I think rather appealing to a wider clientele, perhaps even some of the ladies. The Cigar pen kit can be bought with fairly high-end hardware platings (Black Titanium, and Titanium Gold, to name just two). . So maybe the single barrel version of the Cigar might suit your requirements. . I have posted a pic of a Cigar with a full-length segmented wood barrel. You can view it in my photos.
There really are a lot of options if you put on your innovative/creative cap.
But you need a basic set of skills and perhaps some improved tools to implement these options.
Going through the learning processes (perhaps with standard kit pens) and keeping your senses alert and aware to alternative methods/materials is a first key phase of getting to your objectives of quality high-end slim-ish pens.
The manufacturers of pen kits could really help us out a lot, and sell more pen kits to us, if they would listen to learn what we want.
Improved plating quality would be very welcome, as would improved/innovative design of hardware components. . (NOT most of the fairly recent hardware innovations being sold by PSI). . I think that people who like quality pens would be very willing to pay for any reasonable additional cost of the kits.