Amazon wants to charge you $90/TB* to get data to the outside world, compared to B2's $10, but you can mitigate it in various ways. At the low end that's using a lightsail instance as a VPN, depending whether you think the TOS allows that. At the high end it's paying flexify.io $40 to move your data to B2, then paying B2 $10.

There might be other ways to improve S3 egress costs. It's a very hard thing to search for. I only learned about flexify from this post.

So if you have to restore less than half of your data each year, Glacier Deep storage will save you money. It's worth considering, unlike normal Glacier which is almost entirely downside.

* There's also a $2.50/TB fee to get things out of Glacier, but that's dwarfed by the other costs.

I use this as an offsite backup- that as long as disaster does not strike, I will never use, and even if does, I can be patient about restoring.

This is for home use- which maybe I was wrongly assuming that most Synology users are. In a business context you might have a more routine need to recover backups and those costs become more tangible.