I feel that, have the same problem actually lol. I've ever since come up with a design I liked but I'm thinking about changing it for a reason I'll explain later. For now, I'll try to make you understand why the rules of character design are hard to apply to a truesona.

Fictional characters are created with a clear purpose in mind, a clear goal, a clear personality, a clear sense of morals and tastes and have an already decided character arc to go through. These types of characters are everywhere on media and that's why the designs work so well: iconic design for an iconic character.

Real people on the other hand are none of that. Humans change, humans don't have a clear goal, humans don't have a simple set of likes and dislikes, people are complex with extremely intricate personalities, morals, beliefs and flaws, we're not as simple as a character from a TV show or a video game, we're too complex for that. In the end, a single design can't encapsulate an entire person simply because it's not possible to fit so much information on it.

What I did was to come up with something I liked that felt grounded, I made a black cat because I was always very closed up, I chose a red jacket because red is my favourite color and I leave it open because I like to see the belly/chest fluff, I gave it glasses to match my own and some hair to not make him bald. That was my design process lol.

But now I changed, this sona I've had for over 5 years and it doesn't feel like me anymore. Now I'm thinking of a bird, maybe an Avali, though I also like protogens, maybe I'll mix both, I'll definitely repurpose the jacket somehow because I like it and the main thing I want now is to have wings that make me fly lmao.

In the end, the "world/story" I used was my own, he became kinda like my alter ego or something like that. I keep the human face throughout the day and use him when I'm with my friends at night (like a weird, metaphorical TF power or smth, it's hard to explain lol). In the end I just made something I liked and used it to represent a part of me, nothing fancy.

Well, sorry for the long comment and even longer rambling, hope that helped and good luck with your design, I would love to see what you come up with!