It might help enough if you increase the squish gasket and reduce the compression slightly the load on the starter will be reduced as the compression goes down. (Just an idea) did you hone the cylinders? Get that professionally done, it might help you slightly
Also when I rebuilt my bike I managed to pinch a startor wire when installing the stator cover, I’m not saying this is what you have done but starting my bike was hit and miss until I sorted it! And the regulator is on the way out also not helping with starting issues! When starting the largest load is the starter (obviously) the stator is being cranked as well, it’s not much but kicks out a contributory charge at startup! Just another line of enquiry for you.......oh joy
I should also add that the starter clutch when it’s on the way out can cause the starter motor to draw excess load from the battery. Check the starter clutch for sure! If it goes pop it will most likely damage a whole bunch of other components (mine just did it!) otherwise the damage travels to the starter motor as well....the starter turns the engine over at x amount of low rpm.....what happens when the starter clutch goes is the motor turns over at 8-9 thousand rpm x30 whatever the ratio is in reverse messes the starter the transfer gear and sprag very quickly!
