I can highly recommend an Asrock j5040 board. It is power efficient and enough for all Docker images I throw at it.
The onlydownside is, that it only has 4 SATA ports and 1 pci x1 connector, makes expanding difficult.
Currently running: Nextcloud, vaultwarden, jellyfin, photoprism, immich and a few other container
From a pure performance standpoint a pi zero is good enough to start and learn the basics. Nextcloud and pihole should be working just fine. Regarding Jellyfin depends on what your usecase is.
In any case, I would suggest to start and see for yourself. The you can as you go.
E: To boil it down: Pihole - enough, Nextcloud - very simple file handling, jellyfin - streaming music yes, videos not much