I am getting more sure this is an NFS/networking issue rather than an issue with disks in the server.

I created a small test program that given a directory finds a random file in a random directory three levels below, opens it and reads up to a block (512 Bytes) of data from it and times how long it took to find the file (opendir/readir) and read the block from the file printing the results if the time is greater than previous ones (so seeing the peek times). This is repeated every 10 seconds. First param is the average time to find the file (there may not be a file 3 levels down so it repeats those searches untill it finds one that the user can access), the second is the time it took to find the file (3 x opendir/readdir) to a file that existed. the last time is how long it took to open, read and close the file.

I set one of these processes running on the server starting at the /home dir and did the same on one of my clients that has /home NFS V4 mounted with defaults + async.

The server after 12 hours had peak timings of (file paths hidden):

2021-10-02T09:26:38     0.008858     0.043513     0.031735 /home/...
2021-10-02T09:26:58     0.005384     0.050870     0.039186 /home/...
2021-10-02T09:38:09     0.006684     0.081707     0.014616 /home/...
2021-10-02T10:18:42     0.037394     0.144025     0.012603 /home/...

The client had timings of:

2021-10-02T08:48:45     0.056195     0.110149     0.019353 /home/...
2021-10-02T09:06:31     0.098647     0.098647     0.015171 /home/...
2021-10-02T09:28:38     1.060605     0.001996     0.000422 /home/...
2021-10-02T09:31:28     4.896196     2.037488     0.000836 /home/...
2021-10-02T11:48:44     4.423502     7.087917     1.111684 /home/...
2021-10-02T11:51:02    27.711746    45.646627     0.021321 /home/...

So at one point the NFS mounted client took 45 seconds to find a file (opendir/readdir 3 times) and once before 7.08 seconds with 1.1 seconds to read a block. The actual file it accessed is
46819 Bytes long and can be normally quickly accessed/copied etc.

"sar -d" reported no issues.

"mountstats /home" reported no issues

"/var/log/messages" in both systems reported no issues.

Generally the desktop system has been responsive all day (no other users and nothing obvious going on on both server and client) and I have not noticed a "lockup" on the GUI I have been using (intermittently). No noticeable network errors, no noticeable hard disk read issues, but occasional very long NFS opendir/readdir which would match up with when i see the desktop lock up for around 30secs ore more.