Hi, sorry for the late reply - I didn’t get a notification for some reason. I’m a bit confused - do you mean you got systemd mounting working, or fstab? I think mounting with systemd unit would ultimately be the same as with fstab, but there is a guide for using systemd here if you wanted it still: Manjaro systemd mount units. On your post with mount -a I think your permission was denied because you would need superuser rights to mount that share as shown.
Did you check file transfers using something like dd, just to see whether the issue lies with your file manager? Do you use ACLs? These can incur a performance penalty.