When I NG+ will I have to open the ultimate world shops again ?
Nope, the only thing you lose are the Ultimate Goddess Abilities such as jumping, flying, translating etc so we won't break the story by accessing areas we shouldn't.
I'll start this once I get the rest of my charms to 75%ish or higher ?
I did 15 NG+ runs as soon as I can because that was my instinct from the original Mugen Souls for getting a plat but in your case, only 3-4 NG+ runs are needed. The two final Shampurus give more Power than the entire final tier Shampuru combined
So I'll be peon'ing every post game boss, this mean's I have to do the true end again right ? And how much % will I gain from them ?
Yup, that means get the true ending, don't forget about Dees' chapter, it's really easy to forget to get to the Mugen Field when we're rushing.
What you are after are two Shampuru which are the last two Special types, if you don't have that many then these percentages should increase really fast.
Obvious Multiple NG+ Runs tip (just incase):
For those star points that ask for items, make sure to buy multiple of those items to save time instead of having to search the wiki on how to get them each run.