Yes, you can do that. Or you can add Glacier as bit-identical storage to your repository, and copy the content via and then use lifecycle rules to move that to Glacier. That assuming copy does not need to read the target repository, which I’m not 100% sure of.
Yes, +thawing data 12 hours in advance.
Yes, time, manual labor and money
To access data in the archive you need to thaw it first. Here is time cost right away. See “Data Retrievals”.
Because you don’t know which chunks are used in the revision you are restoring you would likely end up needing to thaw entire archive. This will cost quite a bit. That cost is on top of egress cost when you actually download data. API calls are additional.
Alternatively you could only unfreeze the snapshots file and manually identify chunks they are using and unfreeze those, if Glacier supports that level of granularity, but this involves even more efforts, understanding of the duplicacy storage format and easily avoidable unnecessary labor.
In other words, you are effectively taking up upon yourself manual one-shot implementation of Glacier support for Duplicacy, something that even developers decided to be not worth the effort.
And then there is the question of what all that headache buy you? One less account to maintain at different provider and perhaps a few cents less a month per TB in storage costs. That savings would be immediately eaten away by minus 1000% by Glacier thawing and S3 egress and api costs if you even need to restore anything. Amazon Glacier really does not want you to ever touch that data.
Therefore attempting to use Glacier for backup is more laborious, less reliable (nobody attempted that and there is unknown number of weird issues that await) and more expensive compared to alternatives (some of which have 0 egress cost and comparable monthly storage cost and instant data availability)
Not trying to convince you to go either way, just wanted to point out these non-trivial hurdles associated with unsupported configuration of using Glacier as backup destination.