The five years start next year, so 2024-28. However, I think the SB starts getting prorated starting this year (2023-28) but am not sure about that.
I believe that's correct.
Terence Steele will get his signing bonus now, and it will count over the next 5 seasons, meaning the last year of his extension currently has no signing bonus. The team will likely lower his RFA tender salary, $4.304M, to the 3rd year minimum, which should see minimal change to his 2023 cap number when you factor in the new signing bonus proration.
The Dallas Cowboys get 6 years of player control for $50M guaranteed.