You’re right, what you describe is dynamic content via Ajax. I think that kind of mechanism will be even more difficult to find than the common click a link to get a new page scheme.
What if, using default WP resources, you created various gallery posts where all the images in the gallery are thumbs on the post’s page. Clicking a thumb either opens an attachment page or the large image file itself. You don’t get a lightbox effect by default, but that’s easier to add in than an entire portfolio system.
Place each of these posts in a common category. Requesting that category’s archive will thus list all the gallery posts available. The layout may not be desirable, but at least the information is all there. Depending on the theme, there ought to at least be a featured image and a title. Some additional CSS could hide undesired content and alter the layout to something more desirable, such as an image grid.
The other alternative would be to hire a coder to help you develop a bespoke solution. If there’s already an existing plugin that mostly meets your needs, the cost required to add in additional needs may not be all that expensive. If you don’t already know a coder, professional help can be found at resources like https://jobs.wordpress.net/ or https://jetpack.pro/.
There are other WordPress forums outside of www.ads-software.com, such as over at wordpress.stackexchange.com. However, that one in particular is more developer oriented, though it says it’s also for “administrators”, so I don’t think a question like yours is necessarily off limits.