So I believe the Pi 4 was the 1st to have an actual ethernet controller and not just having essentially a built in USB to ethernet adapter so bandwidth to your HDDs/ethernet shouldn't be a problem.

Streaming directly off of the pi should be tolerable. A bit slower than a full fat computer with tons of ram for caching and CPU power to buffer things. But fine. There's some quirks with usb connected HDDs that makes them a bit slower than they should (still in 2025 UASP isn't a given somehow) But streaming ultimately doesn't need that much bandwidth.

What's going to be unbearable is transcoding. If you're connecting some shitty ass smart TV that only understands like H264 and your videos are 265 then that has to get converted, and that SUCKS. Plex by default also likes to play videos at a lower bitrate sometimes, which means transcoding.

There's also other weird quirks to look out for. Like someone else was (I think) doing exactly what you wanted to do, but no matter what the experience was unbearable. Apparently LVM was somehow too much compute for the pi to handle, and as soon as they switched to raw EXT4 they could stream perfectly fine. I don't remember why this was a problem, but it's just kind of a reminder of how weak these devices actually are compared to "full" computers.