I want to disable iMessages and use iCloud for messages how do I do that

There is no option to do that.

"Messages in Cloud" is simply an option that syncs your iMessages among all devices. You still use iMessages. See the following from Use Messages in iCloud - Apple Support

Use Messages in iCloud

iCloud keeps your entire message history updated and available on all your devices, even when you set up a brand-new device.

Here's how it works

Messages in iCloud are updated automatically, so you always have the same view everywhere you use iMessage. When you delete a message, photo, or conversation on one device, it’s removed from all of your devices. And since all of your attachments are stored in iCloud, you can save space on your device.

You can use Messages in iCloud on your iPhone, iPad, iPod touch, Apple Watch, and Mac. For your privacy, Messages in iCloud is end-to-end encrypted,* which means you can't view or access Messages online via browser.