The rule-of-thumb is three species per foot and try to keep at least a finger's width of space between plant groupings.
I believe the 120 I linked to is around 14 species or so not including the moss, and that's a 4-foot tank. The 180 is a 6-foot tank, so rule of thumb would be 18 species.
Having a few extra species is no big deal, but push it too far and it starts to become difficult to maintain Dutch style aesthetics like distinct groups, contrast, views to the back of the tank and an overall sense of cohesiveness and simplicity.
For now though I wouldn't get too bogged down worrying about number of species. I really only meant that over time you may find yourself desiring to reduce the number of species, because that tends to be the way that it goes when people get into Dutch style aquascaping. Meant no criticism or anything. And like I said, I think 18 is a fine number to start with. Start with a few extra, see what works and pare down eventually as you see fit.
I can almost guarantee that whatever you put down on that piece of paper right now will eventually morph into something different ultimately, as you swap out plants, re-position, change group sizes and shapes, etc. as the scape progresses.
Always good to have a plan though as a starting point. I wish I could offer a detailed critique of your layout as you're seeking, but I really don't have the experience and haven't kept many of those plants. Lots of detailed lessons on contrast, composition, technique, etc. in those journals I linked though, if you're looking for those sorts of tips and are inclined to do some reading. Maybe someone else will chime in.