Oh matey. I literally just went through this and debated putting together a blog post or similar.
I'm not at my computer so I'm typing this from my phone.
The TL;DR: you need to decide whether you'll pay for the security by paying for restore upfront, or when you need it.
Since I yarr most of if not all my content, I did not worry about backing up my TV shows or movies. I take a directory listing of my jellyfin and back that up weekly. Music is small enough that I do back this up.
Cloud cost is abstract and hard to compare apples to apples. But the biggest thing you'll need to decide is how likely will you need to do a cloud restore. The more robust your on-prem backups the less likely you'll need them so I personally went with AWS S3 using rclone. Glacier cold storage is super cheap and for my needs I'm paying roughly $2-3 a month. The catch is if I need a restore, I'll have to pay for the S3 retrieval and then the egress which can be roughly $60 one time.
For companies like Backblaze, you pay roughly $60 annual for about 2TB of hot storage, which includes egress 3 times.
I prefer to save the $40 difference year over year, and instead put that in a budget for a break-glass situation.
In terms of hardware, some people recommend buying different brands with the same storage size. Others recommend spacing out your hard drive purchases so that they don't all fail at the same time. I prefer the latter.
Hope that points you in the right direction