Looking for Advice on Using a NAS for Backups

First, the Synology unit is not wireless, so you are going to need a wired connection to a WiFi access point. And if you're wiring the Synology unit to something in the first place, why not just do wired backups instead of trying to do it over WiFi? And if WiFi, what access point would you be using with the Synology unit?

Second, a TM backup disk should be exclusively TM, no other uses. And if I understand the DS220j specs correctly its internal drives are formatted as EXT4, which could not be used for TM anyway.

As to backup speed, you do understand that WiFi cannot beat wired speeds. I'd also add that in my own use of NAS (a Lacie D2Network2) the responsiveness was so poor even over 1GB wired Ethernet that I gave up using a NAS at all (and I had such high hopes for it ...). All my backups are direct connect drives; at this point they are all USB 3 Gen 2 (10Gbps) and the speed is very nice.

Under ideal conditions, 2.4GHz WiFi can hit 400-600 Mbps and 5.0 GHz WiFi can hit 1300 Mbps. And that's under ideal conditions. Both pale in comparison to USB 3 Gen 2 at 10 Gbps and Thunderbolt at 40 Gbps.

I have limited experience with NAS, but I have used one.

Currently, we have five Macs. All have backup drives attached and all of them are just regular drives. At first they were all magnetic media, but in the last two years we've gone all SSD for a variety of reasons.

None of them will ever backup to a NAS. That's strictly my opinion based on my experience for our Macs. We've had only one drive failure out of a dozen or so since 2015. That's enough to make the simplicity of a discrete HDD or SSD for backup the better choice for us.

I'm sure others will have other opinions and that's a good thing.

I have 2 synology nas and enjoy them a lot but felt for large amount of backup data it took too slow on my wifi network