my DPMS upper has been super reliable right up until now.
it's started to light strike 70% of the time.
the firing pin looks fine, the bolt looks like it's going fully into battery, the firing pin freely rattles back and forth in the bolt, the bolt face is clean, the hammer spring legs ARE on top of the trigger pin, I can drop ammo into the chamber without it hanging up and the rims sit flush...

do I need a extra powerful hammer spring or what? I have no other ideas.

it's doing this with rem GB's and federal lightning. there are both shitty loads but they used to run 100%
I'm using a spikes lower with a CMMG LPK, flat faced hammer.

I would fire 10 shots pushing the bolt fully forward with the forward assist between every shot to ensure it is going into battery each time.

the FA on a DPMS is purely cosmetic

Might be some crap in the firing pin channel keeping it from coming out far enough all the time. How far does the firing pin stick out from the bolt face? have you looked at the spent cases and rounds that didn't fire to compare the strike?

it appears to protrude plenty far
the strike is VERY shallow

I think I'm just going to order a wolff XP hammer spring and see what that does.
I just flushed the whole bolt out with brake cleaner and cleaned the chamber face spotless
it's not striking any deeper, not even when I try a different lower. (testing it with spent brass)

I can not find a single thing mechanical that could be the cause other than the spring

It might be a fluke but check the extractor to make sure it freely moves and is not gunked up.  Make sure the extractor slips over the rim of the case.  Also check the clearance on the side of the chamber where the extractor rest when the bolt it closed.  The metal in extractors can wear or develope burrs that can impede functioning.   You may be right that the hammer spring is just weak enough to inhibit reliable ignition but since it failed to fire on another lower you may have another issue.  If you haven't removed the firing pin I would.  I have seen broken firing pins but no pieces would fall out of the bolt.  Only by taking the pin out of the bolt did I realize the pin had a broken shaft or head.

the bolt face is behind the extractor, so the case head rides up the bolt under the extractor, not over it. if you have tried the upper in a different lower and still have the same issue, yes, as above, check the firing pin spring, and check that the firing pin isn't broken. if that checks out then there may be a buildup of crap or something has gotten lodged in the firing pin channel. the firing pin has a shoulder on it about 1/2" from the tip, and this is where something may be preventing full travel. due to the sharp angle of this shoulder, if anything gets in this area, it will get pounded into the bolt and be difficult to remove. carefully twist a small drill bit with finger pressure to dislodge anything that may be stuck in this area.  on my bolt, at full extension, the tip of the firing pin is even with the face of the bolt. let us know how it goes, good luck.