I moved my websites off of WordPress several years ago due to the security problems I was having and the complexity of customizing themes. There were just too many files in too many places to modify when modifying a theme to suit me. So I switched to Grav and found its straightforward approach was what I needed.
The Squarespace platform allows the web developer a lot of flexibility in creating unique designs. But, naturally, there are limits and some developers will want to go beyond them to create websites that are even more unique. Although not covered by Squarespace’s support team, CSS and JavaScript can be used by more advanced web developers.
Consistent with my shifting attention in web development, I’ve had my photo galleries in all sorts of Content Management Systems (CMSs) over the years. One feature most of those systems shared was the complexity of uploading new images or replacing old ones. My desire to simplify both my web development platform and keeping my images current has led me to Squarespace.