Skill plateaus happen to pretty much everyone eventually. And sometimes it can take a VERY long time to break through a plateau.
The suggestion to play outside your comfort zone is very true. If you're used to playing stuff you struggle to S, play stuff you struggle to A, if you are playing stuff you struggle to A, try to play stuff you can only get solid Bs on. The reason that playing things you're not comfortable with has 2 major benefits.
The first is that it introduces more difficult patterns. One of the key skills in rhythm games is reading patterns. By forcing yourself to play (and therefore read) patterns that are more difficult than your usual is a good way to improve your reading skill, which is sometimes the skill that is holding you back.
The second thing it does is that exposure to harder patterns helps you feel like the stuff you're currently playing is actually easier than you though by comparison (this even works to some degree subconsciously). It also helps teach you to push yourself physically in ways you didn't before.
Another piece of advice is to try to actually figure out what skills are actually holding you back. Before I talk about how to figure this out, I'll explain the basics of what I mean by the different skills involved in these kinds of games:
Reading - Everyone knows what reading is, but most people can't identify when it's reading that's the problem, unless they literally can't understand what they're seeing. Just because you can consciously identify where each note in a pattern is doesn't mean that your brain can do that fast enough to actually hit the pattern correctly.
Speed - Finger movement speed. This isn't an issue for newer players, even if they think it is. Speed becomes a problem a bit later on, when you've truly learned the basics and have the basic skills down pretty well (usually at least several months to a year after starting, depending on your learning speed and how often you play).
Control - Even if you can read a pattern perfectly, and have the finger speed to hit it, some patterns involve difficult movements to coordinate, for example: quadstreams, minijacks, etc. On top of that, the actual transition between different kinds of patterns can require lots of control (for example, switching from stairs or broken stairs in 7k to chord heavy patterns, or chordjacks).
A sign that reading has become an issue is when you feel confident that you should be able to hit a certain pattern in terms of speed, but that for some reason your fingers stumble. Another sign that reading is holding you back is if you are consistently hitting late on a map where you know that the timing is correct. This is one reason why it's a good idea to use the timing meter, rather than the other options.
Sometimes it can be hard to tell whether or not speed is your problem, because reading can cause a very similar kind of problem (for example, making lots of mistakes on a jumpstream can be hard to figure out if it's speed or reading). My favorite way to test my speed is to find a map with fast single note streams. In most cases fast streams are easy enough to read that you can more or less eliminate reading as the cause if you struggle on fast streams (for example, Empress SC has 270 bpm single note streams, although I don't suggest that specific map as a good choice because the stream patterns themselves are designed to be more difficult for players using a layout like the standard osu!mania layout).
Control is probably the easiest one of these 3 to identify. If you struggle at certain transitions between patterns in maps, that's usually a sign that you need to work on your overall control. For example, many maps end a section of jumptrills (12/34/12/34) with a 3 note chord like 12/34/12/34/124. Notice that 4 appears in 2 of those chords in a row. This is a kind of pattern I still struggle to hit, and VERY often miss on, even on maps that are well within my comfort zone. Even at speeds where I'm comfortable and can handle minijack based patterns on fairly well. This tells me that I lack the control to hit that last note.
So my suggestion would be to stop thinking quite so much about exactly what kind of score you should aim for, and play a wide variety of maps, from things you can get 99%+ on to stuff you struggle to pass. Every map you play can tell you more about which skills might be holding you back, and which ones aren't.
EDIT: I want to address one issue that's not entirely related to the question, but might be helpful to know. PP is NOT a good indicator if how quickly your skill is improving. When new players start out, their PP can skyrocket, because of how PP is calculated. You NEED to play enough songs that are hard enough to show your true skill before your PP reaches a level that (VERY) roughly reflects your actual skill level. Once you reach that point, it becomes VERY hard to get more PP. I'm at a point where even though I'm slowly and steadily improving, my PP is hardly moving.
Improving a score on a song barely increases the PP it's worth once your score on it reaches (I believe) somewhere around 700k to 800k. Even if you go from 700k to 900k on that song, you'll get almost no improvement in PP from it.
Take a look at your top scores list (and go down to the bottom and hit that button to load more). If your entire list has a small range between the lowest score and the highest, then you've filled the list and that PP value is as close to describing your skill as it can get. If the range is large, then all you need to do is find more maps that are worth enough PP to score on, and your PP and rank will improve even if your skill isn't. (I just looked at your top scores, and the difference between the lowest and highest isn't huge, but I still think you could gain quite a bit of PP if you played more difficult stuff. Most of your scores look to be S's rather than A's, so I think you could definitely farm some more PP if you went looking for harder stuff to play, if that's something you're interested in.)
Of course, PP and rank are both very flawed ways to try to measure a player's skill for a whole bunch of reasons, so please do not try to use them as a way to measure your skill. Look at your scores on maps, and compare them to scores you got earlier. There will always be some variation, but if you can consistently improve your scores on something, that's a good sign you're getting better (but PLEASE do not replay the same map over and over. Memorizing a map can improve your score on that specific map and look like you're improving, but in reality you're just getting better at that specific map).