Remember, they thinned the Watch with Series 10, probably as thin is they could go while maintaining what they probably consider baseline battery life. So in essence I think you can consider the standard Watch as the Air.
Then you can consider the Ultra as somewhere between or a combination of the standard and Pro.
And the SE is the E.
There probably won't be exact equivalents to all iPhone models any time soon because the products are too different and with different market sizes.