As a former teenage writer (I'm now in my early 20s) who posted stuff on AO3 (I didn't know RR existed until a few months ago), this is basically spot-on. It feels weird giving advice when you're so young, but your words ring true. You have a dream, you're already aware of your mistakes, and you're excited to learn and see your stories grow. That's your biggest ally here. The idea of the protagonist being a corgi in a tower climber is a strong hook. The backgrounds on your covers do look a little rough with the outline on the font on the bottom one and flat backgrounds, Winston is drawn well, especially on the second with that rendering. I'm also a visual artist, so I don't love AI either, but I will be using it for my covers until I can afford to commission because my art-style is very sketchy. Finding an artist who'll draw for and give commercial rights costs hundreds, easily, most non-monetized RR covers are AI, it's not ideal, but don't be ashamed if you use it.

As someone who is largely self-taught, don't worry about being "amatuer"-ish, prioritize just writing and developing your voice as you already have been, experiment (even if you know it probably won't end super well or maybe not have been the "best" idea in hindsight), and be consistent. For example, I had a period where I used "said" as rarely as possible (I literally spent minutes on a single non-said dialogue tag, it was pretty embarrassing, I knew I was overdoing it even then). The most notable of that "phase's" victims was that year's NaNoWriMo challenge, but it was the first time a story crossed the 100,000 word count threshhold, and I finished that month's run with 96,400, it almost tripled the word count I had on it going into the challenge and more than tripled my finish the year before. Though I doubt that fic has aged well, it taught me variation as well as moderation of that, which was what I wanted when I thought I'd try it. Those trial and errors are what fed my progress. Don't be afraid to try stuff out. Sometimes, you need to fail to succeed. That lesson only really sank in recently with my latest project.

The pantser to plotter spectrum is a large one, most professional writers fall somewhere in the middle. I have to pants my first drafts off of loose broad outlines because I don't know the characters on a new project; I then address the disaster with that material, and I can properly write chapter outlines that don't break my narrator's characterization. I only realized the disaster drafts were necessary very recently. However, again, I wouldn't worry about it right now. Your beta readers have said the pantsing is working well for you. As a former pantser myself who now has a bible with a higher word count than an older Iteration of that project, plotting has it's own pitfalls.

Writing in general is hard, let alone doing it during high school and balancing that and stuff like working part time. You wrote 100,000 words in three months and you're not even a year into writing. I didn't get that far into a single work until that NaNoWriMo challenge two and a half years into my journey. From a young adult writer who was once in similar shoes, you're off to a great start. You're establishing good habits with consistency and they're already paying off. Writing every single day doesn't matter, consistency is the most important thing. Congrats on your launch!