Yes, the absence of 3D is very much a choice. I believe this need is already covered pretty well by existing libraries like three.js and threlte etc. These are relatively large and complex to use, because you essentially have to adopt the mental model of having a whole 3D scene or something like that. With Svader, the mental model is just that you have a canvas with an attached fragment shader, which paints the canvas with respect to some input parameters.
The way the reactive rerendering works, is that every time a change in the input parameters is detected it queues a requestAnimationFrame, so yes, it follows the browser refresh rate.
Since Svelte 5 is automatically able to track changes in values, both by reassignment as well as object mutation, anything that changes a value -- no matter if it's designed for Svelte or not -- should be able to integrate smoothly.