At last I found some configuration which uses mainly original size photos without generating derivatives!

My configuration (probably, not the only possible - but this one works!):

Theme: Modus, plugin: Fotorama.

Main options, Photo sizes: L (default) and XXL; changed XXL resolution to 6000x6000 - as my biggest photo;

Fotorama settings:
Backend: Fotorama
Replace picture page (on)
Allows fullscreen: true
Only fullscreen mode (on)
Add close button (on)
Add image info button (on)
How to fit an image: scaledown
Fullscreen navigation style: false
Enables autoplay (off)


With these settings, clicking on a thumbnail opens Fotorama, which uses the original photo (as XXL option is even bigger) and scales it down to the screen size. Right/left buttons and keyboard keys change photos to next/previous. Close button and Esc keyboard key returns to thumbnail view.

The only disadvantage is that Fotorama can display not so much info - just a title or description. To see/edit more, I have to click "I" button and go to a "normal" picture page, so L size derivative is generating. But I don't need it too often - can use Batch manager instead.

Thank you Katryne, I will play more with Bootstrap Darkroom when I have time. If I can make some customizations, I might switch to it. Playing just with settings I could not make it show many small thumbnails on one screen - either the thumbnails are big, ot concentrated in the center of the screen. But that's a different story...