I use NFS on my DS218+ as well, together with FSTAB mounts.
Never looked back.
It takes some effort to setup, but always works and is lightning fast when needed.
Here’s a good read on FSTAB : FSTAB
When right clicking your shared folder with all your content, choose the NFS-permissions tab.
In there you need to point to the specific IP-address from your 4K, otherwise it won’t work.