Home media server 4k multi client

Hi everyone,
I am trying to setup a home media server at my parents home as a christmas gift.
It should be able to share 4K movies on their local network, centralize and secure all their works as well as letting them access those files when not at home.
Here's my "DIY NAS' server build :

- Hardware :
1 laptop
i5-8300H 8 cores
gtx 1050 ti
a 3t 7200rpm external HDD
LAN connected to the router

- Software :

serviio (upnp)

- They have 3 4k Smart TVs :

the main one LAN connected to the router
2 others WIFI connected to a wifi repeater

- Files :
4K 12 to 80G movies

On my first test on the main TV the video playback happened to be quite choppy and I was just playing it on one screen.
So I want to go for a more powerfull build.

I was going for a DIY solution cause I like building things myself and I am in Africa, good quality tech stuff are pretty expensive and difficult to find.
What do you suggest me ?
Buy a NAS ?
Move to plex ?
Should I stay with on an upnp protocol ?

Thanks in advance.