Private and Public fork for workflow

Goal:
My goal is to share some open source code that I am developing but I want to be able to commit to a private repo first then PR the changes across.

Constraints:
The code may contain private information and credentials which are necessary during development but needs to be filtered when each PR goes public. The development commits need to be secure, private and backed up (therefore the necessity for a private fork/branch)
I also use exclusively github desktop for my work and would prefer not to have to learn git command line.

The only way I have seen that this may be possible is by using a mirror and two completely disconnected repos. I can’t see a way to do this in github desktop though

P.S. I am willing to pay for a paid account for an organisation or my personal account in order to get this to work. from my testing it doesn’t seem like paid accounts can have forks that don’t inherit their publicity setting though.

Don’t do that, it’s dangerous. Move credentials to e.g. a configuration file which isn’t committed. That also makes it easier for users of the public code to use their own credentials if needed.

If by “disconnected” you mean one wouldn’t be a fork of the other on GitHub, then yes. You can have as many remote repositories you want registered in your local repository. So you can have one private remote you push to as you work, and another one that only receives what’s ready for the public. See Git - Working with Remotes.