Congrats on making the choice to pursue a midlength! I'm about the same weight as you so happy to share anything I can.
Here's a couple reviews I wrote:
Sunova Carver 5'10" x 20" 85l
www.wouzel.com/post/5-10-x-20-85l-sunova-carver-gorge-reviewSunova Aviator 6'6" x 20" 90l
www.wouzel.com/post/sunova-aviator-18-gorge-wing-reviewI'm currently working on a review for the Pilot which is 5'9"x20" 65.7l
A few things I've learned and noted on winging midlengths:
1. Neutral volume is ideal if you really want to just get up and go on demand. You can go sinker midlength but you don't get the same low end and will have to wait from time to time. Better than sinker chub boards, but still a sinker.
2. Deck shape is critical when getting narrow boards. For an 18" wide board I want an extremely flat deck in order to get command over the rail to rail movement. As such, I don't really recommend 18" for aggressive rail to rail action, it's doable but not ideal if you are using foils with spans over 900mm. 19" and flat is as narrow as I will go in the future. 20" with a slight convex deck is also great fun.
3. Longer than 6' is nice for fast slogging speeds but doesn't get a ton more low end out of your wings.
4. If you can't decide between two boards, get the one that is lightest!
For your options, the Sultan looks really thick, not stoked on that. But, it does have a slightly concave deck that might give better responsiveness at 18" than expected. I've heard these boards are superlight though I don't see any published weights.
The Armstrong is a bit heavier than I would want (I want all of my boards closer to 10lbs or lighter) and also has the raised deck sections which I'm not a fan of. If you've ridden with those already then maybe they wont bother you.
So, I'm a big fan of Sunova. You can customize anything you want on them and my carver which was 5'10"x20" 85l and 10.75lbs was an absolute gem of a board. I replaced it with just a few less liters and .25lbs weight with a 5'11"x20"x77l custom. The pilot I am riding comes in at 9.3lbs and has great low end for the associated volume (much better than the old chub boards) but I do manage to find a hole during each session where I struggle to get back on foil for a short bit. I think it just depends on how fast and how on demand you want your water starts to be! All of the boards I listed here prone paddle great though, super fast for sure.