I also really like the primarchs you made. But the problem with the high damage weapons still stands. In 7th you rarely take more than one wound per attack and with eternal warrior only destroyer hits can inflict multiple wounds. In 8th there are a lot of weapons that do more than one damage per shot. So if you are staring into four lascannons you are more or less dead or severely damaged.
I like the idea of mounty_chris. Maybe with one addendum:
They are characters with more than 10 wounds, they degrade over damage and they can only be shot at, if there is no Legiones Astartes Infantry Unit within 6". So you can bubble wrap them but if there is only a tank nearby, you can gun them down.
Or you go with the other idea, that damage is halved for them to represent their resilience to damage. Or all damage about 3 is always reduced to three? Multiple options.