The following page setup is needed: A block (at the left, see image) with links to node-add forms, which open inline (the area at the right) in the current page.
The reason why the form needs to appear inline, is that this page is part of a group and the layout of the group and group menu and all must stay the same
I tried to create a Panel page variant for the node add form, but the minipanels that I use in node panels are not available there.
I thought about using an iframe, but that is not recommended generally, especially not for mobile use.
I can't find modules that are specifically suitable for this purpose, but I might have missed some.