Ricky Henderson went ok and had an excellent stand out year (2020?) off the back of middling sort of career.Maybe not your exact criteria but plenty f examples to have similarly slow starts and still turned out to be AA.
With that said, I don't think DGB is likely to make it (as a decent AFL player, let alone AA).
Some of the 'slower' starters I can recall off the top of my head that made AA level...(Note: DGB played 28 games in his first 3 seasons)...
Dane Swan played 16 games in his first 3 seasons and was considered awful/close to delisting and was 6xAA
Jobe Watson was close to delisting (many felt he only stayed on the list cos of his dad) and he played 13 games in his first 3 seasons.
Alex Rance played 25 games and was considered an average forward with poor skills.
Not AA but Dylan Moore is Hawthorn's best current example. He was called up for his 10th career game in the final round of his 3rd season for one last go before his planned delisting. He played really well, the first good game of his career, got a lifeline and has never looked back - one of our best players.
Heaps of others tbh.
But the odds are very firmly stacked against DGB unfortunately.