Thanks everybody, for your kind advice! I looked up in duckduckgo about how to set up a NFS4 share in TrueNAS, and found that I just needed to toggle on an Enable NFSv4 switch in the Services panel. Once that had been done, the following mount command worked like a charm!
~> mount -t nfs4 -p “192.168.10.40:/mnt/my_share” /boot/home/Desktop/my_share
Oh well, basically if I’d RTFM before starting to fiddle about with this and that I probably could have gotten NFSv4 sharing to TrueNAS working without all the drama… My apologies!
But if I hadn’t heard from @nephele that might I use NFSv4 sharing instead of SMB I would never have got this far, so thanks again everyone, for all the helpful advice here on the Haiku Forum. 
