Is this stratedy over complicated?

Not at all. You are making decisions based on how long the backup takes and what you may lose if you have a disaster. Every second day is good for your needs.

Is it safe to make backups to Dropbox or Amazon S3, in terms of data safety?

Yes. Both of those services have user agreements about data confidentiality. If you are concerned then perhaps the backups can be encrypted to a passphrase with your plugin.

I backup with a different plugin to Google’s storage space. I’m not overly concerned about my public site’s backup being compromised by spooks. It’d be much easier for spooks to get access to my VPS than my backups. ??

How long is wise or how long does it make sense to keep the backup files?

That depends on your comfort level. If 90 days works for you then that is a good number. Storage is typically inexpensive IMHO.

What plugin do you use for backing up to Google?