quicksync should let the i3 handle jellyfin just fine if you’re not going beyond 1080p for a couple of concurrent users. Especially if you configure the Nice values to prefer jellyfin over immich.

I’m not aware of the platform for the n300 because it might be worth the initial setup, and have some room to upgrade the CPU later if it causes trouble.

If OP is going for multiple systems, I’d definitely agree on making one of them a pure NAS and let a more upgradable system run the chunky stuff.