I don’t understand this opinion that it’s all grey and bland and flat characters.
From my perspective it’s all about character growth, and how movie events affect the characters. For example Han was a soldier in the Imperial navy for three years, probably having inflicted/witnessed much pain and suffering, but this seemingly doesn’t affect the character at all, having the effect of making the character seem shallow, greatly reducing the potential drama of having such a character history. The story focusses on Han, a rather stagnant character, when Qi’Ra seems to have changed the most in the intervening years, changes which mostly happen off-screen. These changes don’t really affect our main character in this movie though, since he remains oblivious to it all. These changes may provide a setup for drama in future movies, but that in of itself doesn’t justify the existence of this one in a dramatic or story sense. To summarize, Solo seems like a low stakes setup for a much more interesting movie, which given the lukewarm reception and poor box office performance may never get made.