By the late 70s/early 80s (when I had some great vacation attachments to Canberra squadrons), all Canberra first tourist pilots were coming from the Hawk - some from Valley (4 FTS), and some after some or all of the TWU Hawk course.
It seemed to me that most (if not all) of the young Canberra pilots I met then spent a tour or two with 7, or the 'Tatty Ton', and then fed back to Brawdy, from where the bulk seemed to be streamed single-seat/fast-jet.
Senior blokes on the Canberra TF squadrons confirmed that one of the Canberra's most useful roles, in this, the twilight of its career, was in giving a bit of extra development to blokes who were FJ-capable, but who had narrowly failed to make the grade on TWU, perhaps 'slow learners' or perhaps having suffered a particular setback.
The Canberra was challenging, and flew a variety of roles (even within the TF squadrons) and clearly helped to turn young pilots who had (for one reason or another) failed training into productive fast jet pilots.
This was a problem, they said, as it made it harder to find Flight Commanders for the Canberra squadrons themselves - since all the first tourists bug.gered off back to the FJ world!
In later years, I became aware that whenever I went to a Harrier or Jaguar Squadron, I'd find a few pilots whose first tour had been on the Canberra - with the Jag Force, in particular, seeming to be riddled with PR9 mates.