how do i share an apple id with my partner but not our imessages?

Have your partner sign out of the Apple ID in the following locations on his devices:

Settings ➜ Account (top of Settings) ➜ Sign out (bottom of the page) It should ask you if you want to keep iCloud data on the device, just answer Yes.

Settings ➜ Account ➜ Media & Purchases ➜ Sign Out.

Settings ➜ Messages ➜ Send & Receive ➜ Apple ID ➜ Sign Out

Settings ➜ FaceTime ➜ Apple ID ➜ Apple ID.

With that done, go back to Settings ➜ Account (top of Settings) and tap Sign in, when presented with the log in box, there should be an option there to "Create a new Apple ID".

More information by clicking here ➜ How to create a new Apple ID - Apple Support

Do note, any apps downloaded to the iPhone using your Apple ID will be forever tied to it, and require it always to update.

You don't. Apple ID's are meant for one person only. Instead, you each have your own Apple ID but set up Family Sharing - Apple

You cannot. You should not share an Apple ID with anyone for any reason.

Your Apple ID is your personal identifier for all things Apple, and as such grants access to all your personal data including your messages.

What exactly are you trying to accomplish? Nothing regarding purchase sharing or subscription access requires you to share your Apple ID.