Even if it does “disappear”, you’d still be able to use it if you have it on your computer. And probably archive.org will have the binary saved.

Is your kindle updated to the latest version? I jailbroke mine and now I use Koreader exclusively.

There’s zonos, and I heard of another one called GPTsovit or something like that, but I haven’t tried that one. Zonos is pretty easy to setup and run though. Another one is Kokoro, search for Kokoro TTS to find it on google.