I just read all of that. It sounds identical to my problem. I have a Asustor As4004T NAS with 2x Seagate IronWolf Pro 10TB NAS Internal Hard Drives and I am getting freezes during movie play. I know it’s not the Raspberry Pi because I have a 2nd media player (WDTV) and it started freezing up as well and it is connected to my network. I just added a Netgear 8 port switch that has everything hooked into it in the same room (TV, Computer, NAS, raspberry PI, Western Digital media player ) and I’m still having freezing problems during movie playback. I was having freezing issues before I bought the netgear 8 port switch.
My cables are CAT 8. I really do think it is the Asustor NAS since it is the common denominator here. I am using NFS in Kodi and also on the NAS but i have not configured fstab based nfs share mounts since you tried it and it did not do anything for you. Article here:Configuring fstab based NFS share mounts
I have been fiddling with all this for days. Just about ready to give up.