the player has to look at the buttons and click
So you have a fixed reticule and you need to aim and click at buttons? Add colliders and raycast to detect hits on the buttons. You can put all of your UI on another layer (maybe name it InWorldUI) and limit your raycasts to that layer and another layer that blocks visibility (so you can't click buttons ...
You can interact with the UI with the cursor locked, but cannot use the mouse, only the keyboard, which is indeed not ideal. We all manage the locking/unlocking in code using a state machine approach so that you lock/unlock the cursor depending on the game state. When the player enters a store you unlock the cursor and lock it again when the player leaves ...
Consider a simple tween where you tween over the scale of something:
already right there you are close to something "popping" up. In general, you can be as simple as much as you can be general about what you want to do to every ...