I have a 1955 GE with the same CRT (a Sylvania 21ZP4, so it was replaced) Mine is also low emission and would need replacement if I choose to restore the set.
While your picture looks as if the main problem is low emission, you may have a voltage issue at the CRT. The other issues like retrace lines can be dealt with later.
Do not forget that this CRT also has an Ion trap magnet that needs to be adjusted or you'll get a dim or no picture. See below from 1956 GE tube data handbook.
21ZP4-B.pdf