It sounds to me like you didn’t understand the details of Svelte’s reactivity model, which are very important, and fairly straightforward to learn, but different from what people are commonly used to—and so even if you theoretically learn the model, it may take time in more complex cases for it to come naturally. The crux of these sorts of problems tends to be that reactivity is a property of bindings, not data; and as a secondary effect, mutation is therefore rather hazardous.
> 50-something reactive variables in a moderately complex chain
It sounds also like you’re fighting the framework. I’ve seen stuff like what you describe in some React libraries (e.g. mirroring something DOMmy in React—common, but I hate it because it’s so much mindless duplication, among other faults), but in Svelte at this scale you’re likely to get better results from seeing if you can bag things together (e.g. let x = {a, b, c} rather than let a, b, c), or passing through components rather than reactive variables. But these are vague concepts which may not actually be relevant or applicable.
Svelte is a framework that definitely requires that you work with it, or you’ll have a miserable time, whereas with React you can, to a much greater extent, do awful things and get away with it.
And when you speak of 3.x and 4.x and such—there’s basically no chance any of this stuff will ever change in Svelte, it’s too fundamental. In some cases you might end up with better compiler warnings, but that’ll likely be the extent of it.