When Miura said "don't get any stronger than this", I don't think he means it literally "stop getting stronger". I think what he means is "it's okay, you don't have to always be strong in front of me" or "it's fine to be weak in front of me". "You've done enough now, it's okay. Let me do the rest, and don't burden yourself with everything," kind of message.
See, he's a blunt person but from what I see, he isn't blunt with what he actually wants to conmunicate一 the core of his reasoning with a lot more of raw emotions一 and just lets you see half of it, if not nothing at all.
But Uka was able to decipher the clues and words he had left, and I think that's what makes their relationship work. They both don't show what they really feel to people when they're really hurt or troubled, and they're both able to tell that and support and depend on each other.
That's at least my interpretation. And I'm sorry, this is so long. I wonder if I phrased it well?
I just don't think of him at that light that he wants her stop growing and be more independent. He wanted her to be stronger and more vocal, that's what started this whole relationship. However, we can see when he hugged her, he looked really pained to see her burst from all her worries after trying to deal with it alone for so long. He wanted to help and lift up her burdens. He wants it so that she'll never feel or make that face ever again.
There are times when you need to be strong but, there are also times where you just have to rely on someone. And I firmly believe that's what Kai wants to do for her.
They both thrive being able to be independent but also need care like succulents. They can't grow with too much water or daily watering but they also need that water and love.
Gosh, this is so long, I'm so sorry but, I just really wanna say it. m(_ _)m