Hi Sean!
I similarly haven't been able to find that icon readily available. It does look like a similar icon exists in the NPS symbology style Steve provided.
Similar, albeit imperfect approaches within ArcGIS Pro would be to use the box tower icon or landmark icon to convey similar meaning, provided they don't compete with other map components.
I was able to review several downloadable styles as well and wasn't able to find a similar icon across preset styles, and styles designed for various industries: Business, Civil, Civil Support, Communications, Gas & Pipelines, Military Emergency Management, Public Signs, Signal Intelligence, or Transportation. Various search terms did not return matches (viewpoint, scenic, outlook, vista, viewing, vantage).
There was an icon that seemed closer in the Government style, downloadable and linked below - the "Laser-Light" icon. However, this doesn't seem to be an entirely appropriate solution either.
In the Government style library, there is a designated "Viewing Area" icon, and a "Wildlife Viewing" icon that seem like the best fit for your needs.
From my research, it looks like your primary concern is valid and I would recommend contributing to the ArcGIS Ideas section of the Community, or reaching out to Esri Support Services for additional support.
