blissey problem!

I will first power him up until it says 5000 or 6000 dust.

Yes power up the P/DG in practice it is better for defence it is something I like to call bating so when a machamp or tyranitar attack a blissey and see pound they are like whatever and dont dodge and then DG comes out of knowhere because if I attack a blissey with my machamp and it has ZH I dodge everything and I stay on top of my dodges but when there is pound I am like yes and that is when DG lowish CD comes into play and hits me for SE damage from out of knowhere.

If CP doesn't matter then I would work towards evolving a Chansey not because I think ZH DG is worth it, but because if CP doesn't matter, then the current Blissey doesn't need powering further, so I'd rather have 2 Blisseys sitting in gyms than only 1.

If CP did matter, then I'd power up the current Blissey because Pound + Dazzling Gleam is still the second best moveset defending moveset and can also be quite good on offense too.

As Tyranitars with B/SE become more prevalent, DG Blisseys will likewise rise above their counterparts. Since he resists both the psychic and normal moves they don't matter as much. But that isn't quite the scenario right now. The only question for you is what CP will get him off that bottom rung in gyms?

Even if CP doesn't matter yet...it will.
It is easier to stay in front of the CP curve than it is to catch up - so power up what you have or you will end up with 2 irrelevant defenders once the CP creep over takes your capacity.