Mostly offtopic, but I just want to drive-by namedrop Legend State, which provides a deep reactivity state system similar to the one in SolidJS or Svelte 5, but with substantially fewer gotchas because its design makes it abundantly clear when you're working with a proxy (which tracks dependencies) and when you're working with plain data.
The key trick is that its "observables" (similar to Solid Stores or Svelte runes) aren't mixed with the actual data, and instead are sort of pointers to the data. This makes it very clear when you're passing reactive observables around, and when you just have vanilla data. There's never confusion about whether a particular property access is properly tracked or not. There's never that feeling that it "is not JavaScript".
It works well with React but is fundamentally framework-independent. We moved to it after trying SolidJS (which Svelte 5 is strongly influenced by) and we haven't looked back. IMO it deserves a lot more mindshare than it currently has.