Hi, growing as it is this tree could be dug with very little effort. They are not deep rooters and growing in the brick box is almost like a container. I would bet you can lift it out of there with a spatula or a flat shovel. They need few roots to carry on, but do gather as much as you can.
I have often started new trees by simply pushing my toppings into the garden and keeping them wet for the first season. Usually the top itself will die back to a lower live limb and that becomes my new start.
Very easy tree to grow with a strong will to live. I have often gathered root suckers from big trees and have several in my garden growing out.
As fall is approaching I would think you could lift the tree in mass and move it to another site if it's interferring with your landscape project. Very good chance of survival down your way. You could also leave it and work with it where it is and collect it after a few years of growing.
If you dock the top it will grow into more of a bush and you can hedge it and shape it. Makes a real nice shrub while it's trunking up.
For the out of towners who have a tough time getting Redwoods I would recommend the Jonsteen Co. They produce and ship global some very nice Redwood starter kits with seed and soil and a little cup to start them in.
In their group on Facebook there are several members from Africa so you might make a connection there for resources. Best of growing, Rick