Over there years I found out that this idea was very popular since 70s and there were many other examples of such "chorded" keyboards - see https://en.wikipedia.org/wiki/Chorded_keyboard
It was Xerox Alto keyset:
It was Microsoft microwriter:
It was the BAT:
and most recent one is SpiffChorder:
Here in 2012 video one of the authors of SpiffChorder describes it and shows how to use it:
https://www.youtube.com/watch?v=7kATKI34jvs&t=659s (as we can see it detects combination when most keys are pressed and sends back code when all keys released - so it's constantly reading all keys to guess what user decided to enter).
Problem with all of those keyboards (as I see it) is that characters are represented by some "random" combinations of keypresses while authors tried to put some logic behind it as to resemble shape of the character for example
(these are very questionable resemblances I think) or some other reasoning as Modified NASA keybindings (used in "The BAT" and SpiffChorder).
I think working directly with ASCII is more logical especially for people who look at ASCII chart for decades and already remember most of the codes :)
And explicit existence of strobe keys in my design should simplify reading algorithm significantly....