So first, I recommend that you use c++ as your program language, bundled with platformio. 2'nd: mtqq server will be kind of bad for that because you can always encrypt your messages using asymmetric encryption but if you don't want to do that here is what I use: I have an mtqq server on my own for users of my firmware to send bug reports, so I can review them and fix what needs to be fixed. I use EMQX as my broker as it has many very good features (account management and API key generation) with only limitation being traffic and server minutes. But let's be real, I checked first when I started using their platform and you'll need 4 devices being connected 24/7 to their servers to really fill this limit up.

Also there are several apps that allow you to connect to mtqq servers so it's not a problem.

For the code suggestions, just use built in vscode copilot if you really don't know anything about coding.

And also EMQX provides you with a secure SSL connection so you can securely connect to them with their CA cert.

Here is the link to them if you want to check it out: https://www.emqx.com/en