This solo is what it's all been about for me.

[shootingstar]

(William, the chorister, is talking with his friend Martin about his solo at Ash Wednesday)

Martin's scowling. 'Last time we tried this trick it didn't go too well.'
'We were stupid ten-year-olds,' William says. 'I'm leaving here soon. This solo is what it's all been about for me. I've got a tiny family. I want them (Uncle Robert and Howard, the two gays) there!' He pauses 'So would Dad (William's late dad).'
'And what then? Your Mum gives them a big hug as we process in?'

(From A Terrible Kindness by Jo Browning Wroe, Part II Cambridge Choir, chapter 29)

I take '. . . what it's all been about for me' to be '. . . what it has all been about for me', right? 'all' should be an adverb there. However, I don't get its meaning, I'm afraid.

 

[SoothingDave]

https://www.collinsdictionary.com/us/dictionary/english/be-what-something-is-all-about

 

[shootingstar]

Thank you very much. So, 'has' is correct there, right?

 

[SoothingDave]

Yes. What it has been all about.