>>340425584
I always like to manually atlas myself. I used part of this guide to learn to do it and it doesn't take long after you're used to it. https://vrcat.club/threads/kattsunes-guide-to-a-manual-texture-atlas-in-blender-2-8.3359/
As for Unity. I duplicate another material for that type of clothing thing from a previous outfit and change the texture and add masks/normal maps/matcaps if I feel like doing some fancier shading.
Dynamic bones you can copy over just using copy component. Some people use this tool for dynamic bones copying and also copying avatar descriptors over from previous outfits. https://github.com/rurre/PumkinsAvatarTools