Time Machine backup to Synology NAS stuck on 'Waiting to complete first backup'
I've got a setup very similar to yours: a 480GB SSD and 1TB HD; both internal (718GB full backup reported in TM). I've been backing up to a DS213 successfully using AFP, and I'm setting up a DS418play using SMB. The Synology forums have conflicting posts regarding making TM work, including the one you reference. Another suggests TM has problems with DSM's SMB implementation, using SAMBA 4.4 (https://forum.synology.com/enu/viewtopic.php?f=64&amp;t=139097). Nevertheless, I have both working for TM backups: AFP on the DS213 and SMB on DS418.

My working config seems the same as yours. Recommend you try these steps:


But, check your DSM SBM settings: 
-> File Services -> SMB -> Advanced Settings 


Max: SMB3; Min: SMB2
Transport Encryption: Auto
Enable Opportunistic Locking and SBM2 lease.
Enable SMB durable handles

Are you connected via Gigabit Ethernet vs wifi? Do this at least for first backup.
Reconnect to your NAS TM folder. 


Turn TM OFF (stop auto backups) 
Unmount the NAS TM drive
Restart Mac
Mount NAS TM drive: CMD-K, enter smb://NameOfNSA/tmFolderName
-- This name/folder should be broadcast via Bonjour from NAS
You should get login window: enter NAS TM user credentials (I
--I use a separate user for TM; Remember pwd in Keychain
Now Select this same "disk" in Time Machine prefs



If TM still won't complete, try..


Start over with NAS TM share


Turn off TM
Unmount drive
Remove the TM backup from DSM Shared Folders
Create a new TM backup using settings above
Restart Mac and setup TM as above

Try pruning down the size of your initial backup. 


See if you can get a backup of just you SSD
Under TM Options, exclude your 1TB drive. 
After successful backup completes, remove exclusion. 
Backup again. 



Even if this works, clone another backup of your drives. (TM is full of strange arcanery -- don't put all your eggs in one basket) 

Update: I periodically see see an error saying "The networked backup disk does not support the required capabilities." In this case, reset the SMB Cache in DSM's SMB advanced settings. This will allow TM to back up again. For a while.