I don't see seams on lowpoly mesh, it looks like that in Blender viewport for example: Maya and Blender have some smoothing settings for UVs, but in Zbrush I am not sure how can it be. For example I have a highpoly and lowpoly meshes, and I do "projectALL" LP on HP or just copy+paste UVs in UVmaster zplugin if necessary. I…
It's likely that your padding amount is too low. 8k will need 64 pixels of padding outside each UV shell, so you need 128 pixels in the gutters between the shells. I would suggest adding more padding, just completely fill the empty gutters with padding. Photopea has a fast padding filter, and you may need to run it a…