@NikoNiko Sorry to hear you're having troubles. There seem to be a few issues occurring at the same time.
1) If you just see a bone (grey circle) without any attachment rendered, this has nothing to do with PMA vs. Straight Alpha workflow. If PMA vs Straight Alpha settings are incorrect, you just receive black or light outlines around attachments, you still could see most of your skeleton correctly. Since you see nothing at all: do you see the skeleton displayed correctly in the SkeletonDataAsset Preview? Perhaps your default skin has no attachments, then you need to switch Skin to your respective skin that has the attachment enabled. Or you have an animation enabled which has all attachments hidden (that is unlikely however).
Could you please share your problematic exported skeleton assets? You can either attach them to a forum posting or send them to contact@esotericsoftware.com, briefly mentioning this forum thread URL so that we know the context.
2) Regarding PMA vs. Straight Alpha: You can find the in-depth documentation here, with all settings shown:
https://zh.esotericsoftware.com/spine-unity#%E9%AB%98%E7%BA%A7---Premultiplied-%E5%92%8C-Straight-Alpha-%E5%AF%BC%E5%85%A5
If you are using Linear color space in your project, you have to use Straight Alpha workflow, you can't use PMA workflow. Unless you have a reason to use Linear color space in your Unity project, it will likely be better for you to switch your project to use Gamma color space. This will make blending more intuitive.
If you need to use Linear color space, you need to adjust all steps in export and import to Straight Alpha, as described in the documentation. When settings are wrong, spine-unity will show error messages that your settings are incorrect.
