Why not just do the same again but for those extra few weeks?

It's slow progression that pays dividends. Slowly add weight, take the necessary rest as you lift heavier and eat enough food.

And I just can't see how you can lose 20lb of fat and gain 20lbs muscle safely and cheaply. Even pros would find that difficult.

Honestly mate, give yourself a realistic target and don't abuse the steroids by taking excessive doses and in time you will lose that fat and gain that muscle.

By time, I'd say a year is realistic. Especially if you want lasting changes.
Stick with compound exercises, eat properly, give yourself a chance to recover and stay clear of injuries.

Hope this helps