Hello ! First question would be : why buy an external drive if you are buying a NAS in the first place ?
Just in case: there are 2 slots available in the NAS you sent, meaning you could buy 2 internal drives for its storage.
On the hosting part, Jellyfin might be able to run judging by the specifications of the NAS. However, you have to take into account if the NAS operating system can run it (maybe there is an app store for it like Synology) and also media transcoding might be limited (to easily stream around your house 4K content for exemple)
Hello ! You might find Sylius suitable. It’s an Open source framework based on Symfony.
Im pretty sure it has all your requirements. The thing is that it’s a headless framework, so a frontend needs to be built on top of that if you want some custom features.
Hope that helps !