More than enough in your opinion. It is perhaps your use case that 4KB works completely fine for, but this is not everyones.

We have a fairly complex set of options users can filter through (by design, and prior to its implementation, by high % of requests by users, FWIW) that need to be sharable on the GET request, they are definitely at least 12KB, likely more.

But that's far too large to easily share in things like plaintext emails or text messages.

If you're unable to compress it further, why don't you implement a TinyURL-type scheme? Save the options on your backend and give them a pointer to the settings. And it can expire after 30d or 1y or never.

Then it actually becomes the size of something shareable.

URLs are resource locators. You're not locating a resource, you're passing a gigantic configuration payload.

This is true, but it is the most efficient way that doesn’t induce complexity like having to serialize every option chosen to the backend immediately. Knowing we can re-create everything in first request is much more simple than other methods and doesn’t require the end user to get a special link generated for sharing they can just share straight for the URL bar.