Correcting kitchen sink drain incorrect slope

So...double sink. Has always been slow, even when clear. Never occurred to me until yesterday. Just happened to have 8" galv nipple stickin out by itself and notice that it slopes toward the trap. Both sides do. Explains excessive buildup and corrosion in nipples that have been noticeable in times past.

So...would like to replace with 1-1/2" ABS DWV. Just cuz. My intent was to glue in the trap arm with correct slope angle. Thought maybe using trap adapter, gluing fixture drain line at a slight offset, but I'm a little stumped the best way to go from behind wall (Galvanized? Cast? 1964 build.) to sink, whole run 1-1/2" ABS...?

Dunno about the tailpiece. Just whatever's available, then 1-1/2 in. ABS DWV Hub x Slip P-Trap? Then...?

My apologies if this is a no brainer, and for any vagueness...

 

If you use tubular piping, you'd put a trap adapter at the wall, push the tubular trap arm in, and tighten the nut to make the seal. Something like this would screw onto a nipple (suggest you change yours to brass first). https://www.amazon.com/Eastman-3540...5?keywords=trap+adapter&qid=1574653550&sr=8-5

 

Hang on tho...the nipple slope, created by the female threaded hole where the nipple threads into...something isn't right there, and a nipple threaded in here is where the incorrect slope originates. That's the entire purpose of my attempt here. To mitigate the apparently 'crooked' nipple hole in the vertical line.

 

How about a pic of your existing set up.

 

The trap adapter at the wall as mentioned above gives you flexibility to set the slope.