Thanks a lot for your answers!
This made me understand everything better.
I updated Jekyll from 3.4 to 3.9 (as was advised via .../_plugins/jekyll_minimagick.rb:7: syntax error, unexpected '<' (SyntaxError) - #2 by MichaelCurrin) and I think, as a result, I was able to follow the tutorial at Using LightGallery in Jekyll without tedious configs without any problem, now that works great.
Will wonder at a later stage how to connect Lazyload to that.