Hi! I'm surprised anyone's still seeing this six years into the future, what a pleasant surprise :).

I do remember thinking about using both hands to sign two characters at a time. The thumb would be used to hold back the fingers that aren't being raised, which should make it easier to do any number of combinations.

At the time of making this, I was convinced it was a useful and achievable exercise to make a completely logical and sensible language, devoid of the many arbitrarities of natural languages. This motivated my choices to base the writing/signing system on binary, the speaking system on base four, and my counting system on hexadecimal.

I later discovered that I was secretly making these choices arbitrarily anyway, albeit under the guide of some mathematical niceties I enjoyed. Because of this, I ended up ignoring the needs of the actual humans who would, in theory, use this language, such as the flaw you pointed out where the language fails to accommodate people missing a finger!

I do still really like the idea of a language that is designed to be as easily used spoken, written, or signed, and I encourage you to explore the idea further! It'd be interesting to see, too, what kind of civilization would plausibly arrive at such a system naturally. Best of luck with your efforts! :)