I use Amazon Glacier for cloud backups because it's super cheap (US$0.004/GB/month). I backup around 300GB and only costs me $1 per month. The problem with the free ones like onedrive/google/etc (which I also have for other reasons) is they only have 10GB or 50GB or whatever but I have more data than this and then costs are higher and you have to pay eg for 1TB and I dont want that much I only have 300GB. With Glacier, you pay only for exactly what you use right down to the last kilobyte.
Bear in mind that retrieval costs are high with glacier. It's designed to backup your data and then never touch it unless your apartment burns down in a fire or something. You can only retrieve 10GB/month for free and then after that they charge a lot (or I have to wait 30 months to retrieve my entire 300GB for free). It's a backup of last resort.
Your privacy considerations are valid, although like shri says many of the worlds largest businesses are running on AWS these days and I think they have pretty decent privacy policv.
Also it works well with Synology and there is a package you can install on your NAS that does the backup automatically. I just set mine to backup once a week on a Sunday. Also set it to preserve originals or keep multiple copies of same file etc, so if I accidentally delete a file on NAS it does not delete on glacier, or if I get ransonware attacked and all my files are encrypted it keeps the previous originals on glacier. It's also quite secure, to link it with your Synology NAS you will need to generate ACCESS KEYS and SECRET KEYS using Amazon website and then put them into your Synology NAS.