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Hi! Took a look at the files and at the lowpoly exported from Painter and wrote down some thoughts (some not on issue):

I think big part of the issue your describing is that the difference of the curvatures between high and low of the gorget is too great - but the highpoly would be best improved as well (see below).

Low Poly

Comparing the density of the different lowpoly meshes, it looks inconsistent, the gorget element being the lightest. I would reduce polycount on some smaller elements (purse strings quite dense) and increase resolution of the gorget, particularly it's curvature. Occluded areas, such as the insides of the gorget can be reduced, or even capped off, depending how you want to use it. Currently, the head opening of the gorget has the same amount of edges as the chest one - in such cases you can split edges as a shape is expanding, with the goal to keep a more even density.



It looks like there are some hard edges without UV splits. When baking the normal map, I expect some visible seams here.


In areas where you have a UV seam along edges with steep angles, like on the seams on the knotted cloth, I would add hard edges. That way the normal map has to compensate less.


Aligning UV borders with the pixel grid will minimize aliasing artifacts. Doing so will likely introduce some stretching. Personally, I would trade stretching for cleaner bakes. For example:

I attached an edited lowpoly with all those points implemented, named 'Accessoirs_low'.

Highpoly

Looking at the highpoly, I see it has hard edges. Recommend to make those smooth, unless you want faceting in your bakes, which become more obvious at higher texture resolutions. With the extrusions/details some edges are so close to each other that the shading will remain fairly sharp. Note that super sharp, un-beveled edges means very tight shading, no shading gradients and therefore the information to be captured by the normal map is very narrow. I would try slightly exaggerated bevels. I think you'd basically want your highpoly to read nicely a various distances, edges having enough width to catch light.

Using a Mid-Poly?

Likely depends much on the specific workflow and subject, but I'd argue in certain cases you might get a better result baking from a more mid-poly with cautiously authored shading. At the same time lowpoly and 'high'poly match more closely. Here is a quick test (at 2k res):
A - baked highpoly (edited some, removed hard edges), textures showing waviness, attached file 'gorget_high_modified'

B - baked a modfied lowpoly (beveled edges/ shading), attached file 'Gorget_high_midLowpoly'

Skewing

I would keep skewing, as a result of baking with averaged vertex normals, in mind. I think the shading of the lower rows of rivets in the screenshots you shared earlier shows this. Based on the cage preview in Painter, you can tell where it's going to happen.

There are several options to address this (and probably more I can't think of currently):

•  Just take it as it is. If it's barely noticeable and you're okay with it.
•  bake an isolated, un-skewed version of that strip and map the game meshes strip UVs to that. (Will also address waviness due to difference between high and low poly mesh curvature)
• modify vertex normals of game mesh or use a cage
• don't bake such details but add them during texturing
• increase geometry (affecting vertex normal) or add such details as geometry

Random notes

With the exported mesh imported, it appears the gorget element is symmetrical, but the mesh is not centered. I would make sure that symmetrical elements are centered so they are easily mirrored or edited symmetrically.


Since the bake settings in the Painter file were quite high (4k with 64x Supersampling ) : When iterating on meshes and bakes, I would do so at a lower texture resolution to increase speed. If you can make it look good, it should look good/better at a higher res.

Well, that took longer than I expected hope it's somewhat understandable. Keep in mind, those notes are based on my experience and others might do things differently. In the end, the result matters. I would certainly recommend the good old 'who put the waviness in my normal map' thread.

Much success with the project!