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.