One downside to Glacier Deep Archive is you pay for at least six months of storage regardless of how long the object is stored, Glacier is three months. That means if you back up data that is deleted within that time you’re over-paying for it. However, given that deep archive is 23X cheaper than standard and 12X cheaper than IA class it probably makes sense. I’m currently putting my data into IA class, but I might move it out to Glacier Deep Archive class.
If you regularly create then delete large amounts of data then you might be best off transitioning to IA class immediately, then moving to Glacier / Deep Archive class after maybe a month. If you create but rarely delete then you might as well push it to Glacier / Deep Archive fairly quickly.
I keep versions of objects in my Restic S3 bucket, to protect against something like a virus overwriting the data. I delete old versions after a year.