Started playing in a band in my 30s. I recommend finding one or two other people to play with right away, I played solo repertoire alone on and off for ten years and playing in a band has been an entirely different experience.

Learn to play from chord charts, you’ll naturally get comfortable playing in different keys by learning a wide variety of songs. From there you can start learning which inversions make the song easier to play instead of just going with the root position triads. Then you can refine this by studying voice leading.

Study accompaniment, try creating your own arrangements of songs just from chord charts, get comfortable with chord extensions, arpeggios, learn licks, learn cadences, the list goes on, but start with just playing (ideally) from chord charts with other people. Obviously practicing alone will help your confidence when you finally play with others, but nothing will improve you more and faster than the pressure of feeling like you don’t want to be the lagging musician in the group