How to symmetrically edit a curve through EndBulge?
Not sure, does pressing shift helps in this case?
In this case, it does not work 
With the control points you have selected, hold down shift (with Gumball enabled) and drag the ‘Scale X’ handle as needed to adjust bulge.
Hello - In this particular case I’d use BlendCrv since it does have the symmetry control, with History if you want to be able to adjust later.
-Pascal
When using Gumball breaks continuity.