Maybe you can try this Is it possible to install only the Nik plugins? - Nik Collection by DxO - DxO Forums
I also didn’t understand why I couldn’t just specify a directory in Afinity where the Nik plugins are located, only with the solution from the link (reinstalling and creating an extra empty directory beforehand) did it work.
I don’t even want to think about why…at Affinity it works anyway.
Have fun