You simply can't base64 large payloads and you're stuck with workarounds.
Are we seriously ok with linking the RFC as source while providing a statement that doesn't match? RFC does matter.
One can infer from the RFC that you can reasonably expect many implementations to fail beyond 8000 characters, and that there are no guarantees up to that either.
True, the RFC doesn't specify a limit, but it does clearly indicate that it's not unbounded, nor should you expect it to be.