Sue,

These examples follow the create, format one, and clone to match the grid size you require; the key attribute is that they transition from a desktop to mobile grid. As each is shared between variants, changing in one variant changes the other:

https://initiostar.co.uk/demo/Photo_Template/

https://initiostar.co.uk/demo/simple_grid/

Any number of variations to this approach.

Number of photos is a design choice, but I would have thought that much more than 20-30 unless each is significantly different would not be viewed.

The performance with the examples seems OK.