When I’m browsing stitch patterns, I tend to try to stick with things that are 40 rows or fewer. That would be to be used starting after a brim of some type, and I’m usually working yarn thicker than DK. So I would say you’ll be fine with even some space above and below. (Me - I’d start the chart two or three rows after the brim, then allow for there to be more space above the art.)
Decreasing - yes, you can figure out how many wedges you want, divide the number of stitches by that, then work as you listed. However, most hats work with wedges of 10-12 stitches in order to get a nice rounded crown. You’ll be knitting decrease rounds quite a while if you start with k2tog, k18 and it might not be shaped as you want. With that many stitches, if you don’t want a lot of wedges, I’d look at doing the type of decrease that gives you more like pie pieces than a swirl. So, with 100 stitches, you’d have four wedges/triangles of 25 and you’d work in paired decreases coming from each side of the triangle. (E.g.: ssk, k21, k2tog, repeat four times for the first round of decreases. If you want 5 wedges, 20 stitches each, so ssk, k16, k2tog for the first round.)
(Rushing and this could be mud. Happy to clarify later.
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