But what if after "we" as a graphic designer made a discussion with the PO, and then the PO want us to create a logo/another visual graphic. Does it counted as we propose to PO too?

As long as you don't do it on behalf of Utopian or with the sole intention to profit from it, no one will ever complain.

Does a Discord server counted as public?

There are better places for such requests, Github is one of them. Have a look here https://libraries.io/explore/help-wanted?labels=design or even better this https://opensourcedesign.net/jobs/