Can I restrict certain Team Members to be 'Read Only' for files, whilst being able to access Posts?

All members have equal access, as you've noted above. If you need to restrict access to specific items, consider putting them into a private channel, or a separate team. Which you can then expose as additional Tab in the team.

You can play with the SharePoint permissions, but as permissions on team sites are not granted on individual user basis, but membership groups, accidents can happen. In other words, it's best to break inheritance... which complicates things. Or you can create a separate library with custom permissions, which brings us back to the above suggestion.

Sensitivity labels are another option.