" am thinking which microcontroller i should try next"
One of those you have already tried!

You will never become proficient in the idiosyncrasies of any one family if you spread yourself too thin. I have worked with the XA now for 5 years and still learn things that I would know by hart about the '51 when using it.
Get good at one or 2 families, then gett better at those.
You have tried the '51, try another '51 derivative e.g. one with a/d or d/a to explore new worlds and at the same time learn more about the world you know.

Happu hollidays,

Erik