How do I control what music is on what device? I do not want to see my wife’s music on my device and vice versa

As noted above, we have different iCloud accounts already. So I would need to create an Apple ID for my wife?

Each user should have their own Apple ID, but normally does not need more than one Apple ID account. A single Apple ID account can be used for both iCloud and iTunes.

But what happens to the music she already has on her device?

What happens to the music you already had on your devices when you subscribe to Apple Music, is outlined here:

Access your music collection on all of your devices with Apple Music - Apple Support

Basically, music on mobile devices gets matched to existing music in the catalog. If a match can’t be found, then the user gets a personal copy in iCloud (not counted against other iCloud plan usage).

When you turn on iCloud Music Library, you may be asked to keep existing music (merged), or replace it with the iCloud version. Not sure if that also applies to single device accounts.

Back up your music collections before you start, to be certain to not lose anything pre-existing.

we have different iCloud accounts but the same iTunes account.

What does that mean, to share the same iTunes account?

Do you subscribe to Apple Music streaming service? Do you want to? Do you want to take advantage of the Family group subscription?

With Apple Music, each user will get their own iCloud Music Library, on their own Apple ID account. Even for the Family subscription. You are only linked to each other by the access to the service, but not the content that you enter into you library.

Family Sharing - Apple Support

Set up Family Sharing - Apple Support

Get an Apple Music Family subscription - Apple Support