Hard to tell from a small pic, OP, but it looks pretty steep. The steeper it is, the more soil and substrate will have to be moved to level it.
I think that to reduce that slope, from the retaining wall onwards, and just moving it to the higher area, will mean that the top part will have to be on a much steeper slope than it is now. That will make it bloody awkward to mow or strim and difficult to use.
My advice (which is probably a waste of key strokes, as you've ignored pretty much all advice up to now) would be to have the spoil from levelling the lower area removed and disposed of. Then you can keep the top part pretty much as it is, and build a retaining wall, possibly using railway sleepers to reduce the cost as brick would be expensive. It will need to be very strong, because the weight of the material it's retaining, plus rainwater, will be considerable.
Not doing this properly will risk the whole lot slipping down on the area below.
Where I live there were houses built at the foot of the mound on which a medieval castle sits. One day, several houses ended up with vast amounts of the castle mound right outside their back doors, despite all the work having been properly done and overseen by experts, and approved by English Heritage. They had to move out for months!
You don't want similar happening on to your nice new patio!