Lowering the rear or raising the front will increase the rake angle which should make any bike more stable at speed but turn left/right slower i.e. you will have to input more motion into the handlebars to get the bike to turn.
Not bad, you kind of want a powerful upright bike to be like that anyway.
I would bet changing handlebar width would countercorrect for that.
Not sure where I am headed next but one option is to take my Schwinn Axum 29er frame and put a 24" wheel with a Leaf motor in it in the rear and a 26" wheel on the front. I also have a crank that has very short crank arms that I would install.
You'll have a hard time keeping the front wheel on the ground even with a 26" leaf at above 4kw. Whatever geometry alteration you make, make sure it allows you to lean on the handlebars so you can baby nudge the throttle.
Right now it has a 26" rear wheel with a Leaf motor in it and a 29" front wheel. I plan to leave it like that until I get a chance to try it for a little while. Motor/wheel mounted, still need to mount the controller and battery plus wire everything up...to be continued .
I'd recommend you move the battery as far forward as possible to help counterbalance the wheelie power of this motor.