There was always a nagging voice in the back of my head - "their backup encryption can't possibly be good" - but I wanted to believe it since their deal is so good, in terms of storage per dollar, and it's hard for me to afford otherwise right now. I did a bit of digging not too far back and their help pages go into a small amount of detail but ultimately just say "trust us". I trusted them to throw away my encryption password after a session, which is terrible, of course. After reading all of your work, I cannot even countenance trusting them anymore.
Ironically the first thing that comes to my mind for offsite backup is Backblaze B2... if you do all the encryption before it leaves your machine, then it doesn't matter what they do, and their storage is cheap. Rclone or similar means that their client software need not be trusted. It's up to whether you tolerate their business practices.
You mentioned in another comment that you are still looking for cloud backup software which ticks all the boxes - why did you not go with Rclone? On the surface it looks great but I haven't tried it yet.