I have posted an example as a user tip here: How to Organise the Hidden Photos in Phot… - Apple Community
Smart albums are one of the best features of Photos, it is worth to explore them. With standard albums we have to add the photos manually, with can be a lot of work, as we have to find and to select the photos, then to add them to the album.
A smart album will select a subset of photos automatically, according to the rules we specify for the album, and it will automatically be updated.
When a smart album contains the rule “Photo is hidden“, the photos in this smart album will always be hidden. We could hide them in the album by changing the rule “Photo is not hidden”. I am simply putting my smart albums with hidden photos in a folder and close this folder in the sidebar, so they are not accidentally opened.
Filters are also great, particularly the filters based on keywords. If you assign keywords to your hidden photos, describing the kind of photo, for example “medical” or “private”, you can set the filter to show only photos with this keyword, see for example How to use the Filters in Photos 4.0 - Apple Community