Dear @Konrad Woloszyn ,
afaik this is not possible. The recommended way is to open the space for all but creating two top level child pages eg. public and internal.
internal is restricted. If you would like to make pages visible to others, move them as children below public.
In addition to the answer by @Thomas Deiler If you want to maintain the structure in your 'restricted'/ internal page tree, I would suggest the include page macro - https://confluence.atlassian.com/confcloud/include-page-macro-724765214.html
You can then create your public content in the public tree, and then create an empty child page of the restricted page which has the page include macro to pull in the public content.
Hey @Konrad Woloszyn , yes it will work cross space.
I recommend that you familiarise yourself with https://confluence.atlassian.com/confcloud/macros-724765157.html , that will give you all the details you need on the macros available and how they work.
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