Interesting thread, and a good read with lots of great information provided for the OP. I, too, learned to ski as an adult and have kept at it for the better parts of three decades.
My take on the scenario: OP tried friend's Magnums, had fun on them, and is now concerned that his own skis are holding his progression back. Personally, I have progressed on every ski I have owned, whether it was narrow, wide, stiff, or soft. After a number seasons on a pair, I'm ready for something different. A new ski does allow me to experience the sport in a different way and improve a particular aspect of my overall skiing. But, in reality, it is most likely the accumulation of hours on the snow, experiencing different conditions and terrain, that contributes most to my overall progression.
The advice to demo many models is spot on. Find a ski that meets your current needs and that makes you excited to be on the mountain. For the OP, this could be a more demanding ski that forces him to maintain proper technique, or it could be a finesse ski that gives him the courage to explore new terrain. It could be a wider, rockered model that excels in free-ride conditions, or a carve-oriented model to master the groomers. Or most likely, it will be a model that combines more than one of these attributes.
Almost any ski can advance your level, if it keeps you excited about the sport and on the mountain. Get out there, challenge yourself, have fun, enjoy the ride.