I think your art style is cute! I think the best way to level up is to continue fundamentals. Mainly just proportions and construction. No amount of rendering can fix a bad foundation. At first glance, it looks like the eyes are too low or the head is way to big. And the neck is too wide. The torso is also a bit big compared to Furina's official art. For rendering specifically, i think it'll be good to focus on what rendering style you like and have that as a clear reference. If you like the genshin impact rendering, it'll be good to have a few characters beside you to check how they render the shadow and light. But a good way to learn is to go simpler. You have many values that you're messing with, but the values are muddy. You need to have a clear separation of what is light ad what is shadow. All of your values are pretty light. I would practice with simpler rendering styles like from anime so that you can practice the shadow shapes.
maybe keep one character study in two flat value shapes first, then add the soft shading back in, so the light side and shadow side stay clear
maybe try a quick black and white pass first, keeping the head, neck, and torso as simple shapes, then add just one clear shadow side so the values stay separate
yeah the proportion note clicks, maybe block the head and torso as simple shapes first and check the eyes and neck against Furina before doing another render pass
maybe block the head and torso as two simple shapes first, then check the eye line before adding another render pass
idk if more rendering is the first fix, maybe check head size and eye height against an official furina ref first, then keep the light shapes simpler so the values feel less muddy
maybe the head size is part of what makes it cute, idk if you need to shrink it hard. locking a cleaner light and shadow split on the face first might fix the muddy feel while you keep the style.
idk if you need a fully simpler style yet, maybe keep the soft look and just push one clearer dark on the cheek and hair shadow so light and shadow split without muddying the mids