Not 100% sure but I think I read here (or somewhere else) that once you have extended your lease, you’re actually still free to change car at any point, so extending to 5 years doesn’t take away any options – if you find something in 6 months, you can order it.
The good condition bonus is based on full years though so you’re limited to (at current rates) £600 if you change between the end of year 3 and the end of year 4, it then increases to £750 until the end of the 4th year, and rises to the £900 at the end of year 5. If you change (actually take delivery of the new car) at 4 months, 11 months and 3 weeks, you only get the 4 year bonus.
I have no idea if you have any walk-away cancellation options between the end of year 3 and the end of year 5 though. You can cancel to take a new car on the scheme but I don’t know about just walking away
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."