Unfortunately, as you mentioned, permissions are not at the post-level but forum-level. Additionally, the software was not built to have content in 2 places for 2 different userbases (as Marc mentioned. I am not aware of a way to accomplish this via third party existing tools so this might need to be custom if its something you need).
Therefore, you have two options really:
You tailor your forums to what you want your guests to see. If it's content X but not content A, B, C... then that would need to be tailored in your forum category decisions.
If you don't want to give guests access to all content in a forum, you may wish to not allow guest access at all but rather provide free trials. Then via Pages (our CMS application) write your own marketing content which would entice (based on some sample content that you screenshot or copy into your marketing material) your guests to go sign up for a trial and that free or minimally priced trial will expire so you are not providing endless content to anyone forever. This option would not, however, allow you to post topics to social media directly because guests would need to login to read it but would allow you to promote the content via your marketing material.
There are some third party add-ons out there (Please note, these would only be supported by their authors so you would want to contact them to verify their features and their support):
That go a bit deeper with permissions and put them on the topics.
Limit the number of topics a guest can view. While this may be enticing, I would not recommend this as if you intend to leverage guest viewing so that search engines can index your content. As this will impact you negatively.