Thanks @lauthals for your feedback - I incorporated your ctrl+shift improvement to my setup.
Please have a look at the function layer - fun+shift is what you are looking for.
Ah, I see. I actually missed that. However that still feels more like a workaround, because, especially for the shift+mouse click scenario, I would have to do a pretty weird gesture with my left hand.
I will post, if I come up with a better idea. Thank you so far 
hello everyone. im new here and just learning to type with my new atreus.
Im not a programmer, can someone tell me how i can install sekos firmware? looks very promising. would be very appreciated 