Good morning Richard,
First cut: well done! You have started.
On the second cut: You understood us well.
Allways look twice and cut once. So I took a good look at all your pictures again. Just above the yellow line, on the right side, I see two thin branches. In another picture it looks like at least one of these branches is growing in a different direction than the thick one on the right. If so, you migth want to save one of these thin branches (and cut the other one away). Having 3 branches spreading in 3 different directions would be a nice start of a new, more balanced tree. However, if they grow in the same direction as the thick branch, you have to cut below these thin branches, or it will cause trouble in the future.
On the exact direction of the cut: the goal is a nice clean cut, without ripped bark. You want all damage to be removed completely. You want an inclined surface from which the water can roll of easilly. The strongest regrowth will happen at the top of the cut. So,
- If you are going to cut above the thin branch, I would make this my highest point. That would mean, if you would take a picture from the right side of your tree, you could draw the yellow line best. Around the thin branch, there will be a collar of bark that looks a little different in structure. You don't want to cut into that.
- If you decide to cut below the thin branch, I would cut a little lower than you draw the line or, even better: make up my mind on what direction I would like the new top branch to grow and cut from there downwards. That could well be just below the end of the tear. There will be sleeping buds that wake up when you cut there, and make a new branch/top of the tree.
It is probably wise to reduce the height of the tree first with a higher, random cut, so the weigth of the tree won't do any more harm when falling down.
When the final cut is made, return on a regular base and remove, just with your fingers, all excess growth, and keep only one new branch at the top. Imagine all new growth becoming thick branches in the future, and remove now whatever will cause trouble in the future, so you will never have to do something this drastic again.
And don't forget that picture at the end of the year! :)