Do the DLC before accepting Preston's offer of becoming the General. You can rescue their group, you can sort of rebuild Sanctuary, you can do Preston's first quest. As long as you don't accept the offer, you can do the entirety of the Nuka World DLC without worry. If you don't know when is it exactly, it's when you can have him as a companion.

I did this. Just now. Actually, Power Play has been finished since yesterday but I still had to grind Gage's friendship points for his friendship perk(go to Green Trailer Estates for easy points, with the safe and terminal that locks it).

I'm still feeling chatty. My headcanon for my Survivor going along with the Raiders is that it's too dangerous for them and the traders to just go all out with all three gangs in full power. As long as I keep them in check and not hurt too many settlers, there might be a time where the balance of power might shift towards something advantageous to taking them out. And Power Play provides that moment...