After some trial period I came to conclusion that SMB broadcast disappears just over a time and not because of NFS. NFS remains disabled, I've disabled SMB service in Synology box and within less than half an hour Freenas is gone from Finder. It's enough just to save the same settings in SMB service in Freenas to get it reappeared in Finder but like I said, it's only for short period of time.
Why it's like that? Can't be my network since Synology SMB broadcast is rock solid across all subnets...
The only one difference is Freenas has 2 interfaces used for SMB: one physical igb1 (passed through by ESXi) and second virtual vmx1 (to share physical nic with other VMs). I keep toggling them on/off to see difference but nothing comes up yet.
igb2 is used for jails (i.e. iocage create -r 11.3-RELEASE ip4_addr="igb2|172.16.1.3/26" defaultrouter="172.16.1.1") to assign various IPs in subnet it is connected to - can't achieve that in GUI.
vmx0 is iscsi target for ESXi.
BTW Netbios name (freenas_smb) set in SMB service is not respected and I can see FreeNAS as hostname (freenas).
How to get SMB/Netbios broadcast to be reliable?