So the Storage Lead Developer of Scaleway reached to me saying that they had real issues with buckets with more than 1m+ objects, the databases can't handle it. He said my bucket was really stuck so he couldn't do anything.
They know I'm migrating so he's keeping a eye on it and we keep in touch while it's ongoing.
They will offer this month's bill, which is cool.
Sometimes stuff goes wrong, 
Migration status:
980k objects / 1.8m
409 GB / 699 GB
75% as of now. The majority of what's left is broken objects so it will take longer
Stuck at 85%. About 250k objects are missing but their metadata is supposedly broken. I'll see tomorrow with the provider
The thing is, the background jobs queue is huge now, because there is a backlog of 3.5 millions jobs. So Redis is using 5-6GB of RAM which is messing up with the server.
Since this is getting pretty bad, I decided to switch to the OVH bucket with the missing files. I would have preferred not to but it's the best option right now.
Background jobs are processing right now, faster and with less errors because the bucket is usable. From what I see the number of jobs is still going up. Let's wait and see.
I disabled full text search so that I could turn off Elasticsearch, because I'm very short on RAM.
sidekiq is only processing 50 jobs at once, increasing it seems to exceed the pool of postgres connections
Well I've raised it to 100
@admin Congratulations on the progress!
@admin I often refer to myself as an "IT guy", but I have no idea what you are doing right now 
Wow, it's 2x to 6x more efficient. The backlog size is finally going down!