So this will be a big post as I have a few things to go over.
When you stack UVs you must offset them to the next UV space prior to baking otherwise you will get artifacts.
These are some of your stacked:
Those triangles are because of your stacked UV shells when baking:
Offsetting means selecting the stack and pressing G + 2 in Blender to move them over.
After baking you can bring them back.
You also need to watch your overlapping. Here are two UV Shells that overlap.
Normally when you get such problems look at your 2D view in Painter and go back into your 3D modeling application and review those troubled spots:
If I select all overlapping UVs I get:
Another issue is you have a lot of separate pieces all combined into one.
So when you bake you'll get these artifacts:
You need to split all your pieces and name them whatever_low, then duplicate that and change low to high for High to Low Poly. As you're doing just a single low you should be able to just do _low and when you bake select Always and it should be fine using the High as Low, but otherwise do the below:
Then when you go to bake load your high fbx file and set these options:
Once you bake you'll notice another issue even though now your current baked version is better than your prior:
BEFORE:
AFTER:
The part which overlaps is causing an issue:
I would just split the object so even though its overlapping they're two pieces and the problem will go away.
You also might notice stuff like this on your normal map:
You'll need to review the geometry to find out why. The best way is to paint a color on the UV where that spot is and look in the 3D view where it matches, or reference your 2D view to your UV veiw.
Once done you'll get a better result:
