Do you really want to use GrapheneOS? If you don’t mind using something else, the other operating systems I mentioned support devices for a lot longer. And if you’re not using this as a main device for payments, photos, messaging, banking, etc, then there are less risks in terms of security and you could even buy something a bit older than the 4a.
In terms of video streaming, the Samsung Galaxy S9, S9+ and Note 9 all have beautiful 1440p AMOLED displays with no notch. I still use a Note 9 myself and it’s really enjoyable to watch videos on. I even used to watch movies on it before I bought a tablet. Those three phones have great custom operating system support (no Graphene or Calyx though) and were very popular back in the day so there are lots available to buy secondhand. If you are buying this as a main phone, I would maybe recommend against it due to their age but for a second device they would be excellent (assuming you can find one for close to your budget).
EDIT: I should mention those three phones also have SD card slots and headphone jacks, which makes them even better again as media consumption devices.