Patching and Plastering - Drywall Celling

I have about a 500 sq ft area that I removed popcorn, drop celling, and removed a wall to open up the space. Some of the drywall is new, and appears to be wavy. So far just the joints are taped. This area is open and your able to see majority of it with out obstuction, so Im worried about noticing imperfections. Whats the best method for "leveling" or getting the finish uniform enough so you wont notice once its painted? Im still trying to find a decent finisher in the tampa area, but most dont do residential, or wont deal with the home owner. Im eventually gonna run out of time, and have to do it my self. So Im trying to prepare for the worst case scenario

Can you determine why some of the drywall is wavy? Was it hung incorrectly? Have you set a straight edge on it?

Skim coating is the best way to get a nice slick finish.

By "wavy" do you mean "pillowing", where the gypsum board is deflecting down between the joists?

Not enough screws were put in, and humidity may have caused sags/waves? Or too thick of insulation in cavities? Or uneven joists or studs where they had the crown in the lumber going every which way? Or a possible remodel of wall sections where they covered over old doors or windows or something and did not get the wall on a flat plane?(Or are we talking ceiling problem?)

Its mainly the truss's causing the issue, against an exterior wall there's 3 truss's that have metal plate's that stick out more than others. So this is the culprit in this area. Im going to try and mark all the high spots and tie the 2 truss's in the area together and install additional screws to see if that helps.

You mean 'gang nail plates'? - that were installed perhaps too low on the sides of the 2X's?, instead of being flush?

Yea, the 'gang nail' plate's on 3 truss's were nailed to low