Force/Change number of column for summary block (wall design-template 7.0)

Site URL: https://www.raconte-moidessalades.com/mosaique

Hello,
I created a page to show all my posts at once... Was happy to find a way to counter the 30 limit AND have a nice looking wall of pictures by creating myself a model for 30 images that ended at the same level so I could keep going on "forever":

...only I hadn't seen yet that you could only set the summary images by width size and not a number of columns. 😥

On my rather big desktop screen (32"), it displays 6 columns, so there is weird gaps between two summary blocks (obviously)
> I would like to set it at 5 if possible by adding some css

And on the other hand, on mobile, it shows only 1 image per row, but since I have a lot of pictures (and it's only gonna keep growing), I would like to set it at 2

Here is how it looks with 5 per row:

Not yet a full year (close though) and already 96 posts

I can do really easy code (like point to a block and change the size, color or font), but this is too complicated for me. 🙄

If anyone can help with a CSS code, it would be SOOOO much appreciated:
Either one code to set the desktop display on 5 columns and another for 2 columns on mobile
Or maybe there is a way to simply set the minimum of columns to 2 and the maximum at 5 (let it do what it wants in between... I'm not gonna be able to control it anyway, and I don't really care -a very low percentage of viewers on tablet-)

Here is the page I refer to: https://www.raconte-moidessalades.com/mosaique
Also, I have a personal plan, so I can't inject code in the header of the page but only on the design page.

Hopping to find a nice helper here 😁,
Have a good day,
Letizia