Actually it definitely is possible to self host plausible analytics! It's an open source project with an AGPL license. You can even find instructions for self hosting here: https://plausible.io/docs/self-hosting

Please notice that although they recommend docker for self hosting, it is very easy to self host an a bare RHEL server (you need to be a little familiar with Elixir though but not so much, you can take a peek at the docker entrypoint scripts to see what's going on) ; I'm doing exactly that for some months now and I'm happy with the results.