Heather, as you can see, opinions and suggestions run a wide range, so let me try to give you a few different things to think about to either help or hinder! Grab a snack and a drink, it's a long one!!!
Your current boat is stable, but slow. This is a common combination, and you should understand that, for the most part, a faster boat will be less stable than you are used to. However, with a little bit of practice and patience, you should get used to it. Therefore, when demoing boats, don't let any feelings of twitchiness or instability affect your decisions too much. This is just a natural step of the development process as a paddler.
You have done a lot of overnight camping trips already (assuming in the Sedna?), so any boat with an equal or larger storage capacity will suffice for continuing overnight trips. If you are looking to more extended trips, the only extra room you might need is for the additional food, water, and maybe an extra change of clothing. That will not take up much additional room compared to the amount of gear that is always needed for ANY camping trip, whether it is for one night or many. Even just an upgrade to a more packable tent or sleeping bag can offset the additional space needed. Generally speaking, going to a 16+' boat from your current 15' will probably have more storage space.
You are looking for a boat that is "pretty fast" - but to the people on this forum, that can mean many different things. Are you just comparing to the Sedna, which is probably making it hard to keep up with other paddlers, or do you want to really cover some miles!! If you are just looking to go faster than the Sedna, then most any longer and/or narrower sea kayak will suffice. There is also differences in hull configurations that can get pretty technical, but basically a boat with a greater taper from the bow to the cockpit should be faster. Boats with vertical or "plumb" bow (think butter knife front) like many of the Rockpool boats, will really slice through the water and be a very fast boat, but with that high speed comes some sacrifice of maneuverability. Some might argue this, and say they can maneuver their boats just fine, but it is basic boat design physics.
You ask for a boat that is "pretty stable", but that again is perception. If you want a boat faster than the Sedna, it will be less stable. The question you need to ask yourself is, how much less stability are you willing to work with in order to get the speed you want? As I said before, you should be able to get used to it, but it is just a matter of how much effort and time you want to put it.
Ruddered boats is a big area of controversy. Many sea kayakers view boats with rudders as "cheater" boats, or for the less-experienced paddlers. That is, until you get into specialized boats like surf skis, then rudders are ok? My thoughts about rudders (and skegs) are that they are a tool. Period. Different tools have different uses. Boats with rudders generally need them to turn because their design does not lend towards turning the boat just by leaning or "edging". I would suggest you look further into what the differences are, how each work and why. Then make a decision on whether or not you want to stick with a rudder. My only advise is,, don't just stick with something because you are used to it.
I will combine the ruggedness (dragging over rocks) with weight, as it is a mixed conversation. Basically, plastic is most rugged (nearly indestructible aside from warping), and typically heaviest (although can be similar weight to some "expedition" fiberglass boats with additional coats of glass and resin). Carbon fiber is stiff, but brittle and challenging to repair, but also the lightest. Fiberglass is most common, being a little lighter (in general) to plastic, a little flexible (but can develop cosmetic spider cracks), and relatively easy to repair.
I don't recommend plastic because of the weight and speed (sorry everyone, but I believe that, taking direct comparisons, plastic is slower than fiberglass or carbon). I would also be against carbon fiber for you because you don't want to be dragging that over rocks. One option is to get a "50/50" which has fiberglass bottom and carbon fiber top, but that is typically a custom-ordered boat and might be hard to come by right now. I don't think you will go wrong with an all fiberglass boat.
Overall, I think your budget is fine, unless you start looking at brand new custom-built 50/50s or expedition layups. Other options like custom colors, sparkles, or a select number of manufacturers can add to the expense. Buying a used boat has the advantage of getting something that you can easily sell again if it just doesn't work out for you, typically near the same value you paid, or at least for less than the cost of renting a couple of times. You can always opt to sell and buy a new version if you fall in love with the model.
Sorry this is so long, but hopefully it was a little educational. Don't be afraid to continue reaching out to the club for help. Many people have been willing (at least pre-pandemic, and probably will soon again) to let others try out boats to get a feel for them, so it never hurts to at least ask. This is especially true if someone might be thinking of selling their current boat.