EDIT: It appears this may not in fact be a Synology issue, as I'm now experiencing the same behaviour trying to run a Time Machine backup to a newly-formatted local USB drive.
I just bought a Synology DS220j mostly to replace my dying Airport Extreme, which I was using for Time Machine backups from my MacBook Pro (10.14.6). So far, I've been unable to complete a single TM backup to the Synology.
The main problem is that the backup always stops after backing up only a few tens or hundreds of megabytes, usually with no errors in the Time Machine preferences pane or in the Console, but very occasionally with a "Could not complete backup" notification.
I've tried doing a new backup from scratch, and I've tried seeding a backup from my last good sparsebundle file on my previous backup disk (after fsck-ing it for errors). I've tried over SMB, and I've tried over AFP (which I've now turned off again, as I know it's deprecated and generally doesn't play too well with Synology).
Currently, I'm running a cron job on my Mac that runs 'tmutil startbackup' every 5 minutes. This is my second attempt at that—the first time, it nearly got to the end after a couple of days, but then the "amount remaining to backup" just kept increasing faster than it was backing up, so that backup never finished either. (I don't think that's a Synology problem, though—seems to be a known issue when using TM with a NAS, though I don't know how to solve that either.)
Can anyone suggest anything to make this less painful, and preferably, Just Work?