Yeah, I have two, and there is no way to put some proper OS on them :~((.
I don't know if it's still maintained, but I used to build this one from source:
https://legacyxperia.github.io/
If you're curious about the possibilities, it might be worth searching xdaforums for "mango", and browsing threads like this one:
https://xdaforums.com/t/is-an-xperia-mini-still-usable-in-20...
I haven't bothered, mainly because 3G service is disappearing nowadays.
Will, check it out, thx.
I miss that phone.