> Heck, there's also a symbol for twenty, 廿, and it exists despite the number system being base 10.
I'm not sure that if you intend to captured the structure of chinese symbols, you wouldn't to make the radicals something below symbols. Then 廿 wouldn't count as a single symbol here, but rather as a concatenation of two 十, i.e. the same as XX is 20 in roman numerals.
You are wrong about this. The structure of the characters is very explicit; one character per square. Meaning is assigned at that level and never below.
(As a matter of character etymology, you are right that 廿 and its variant form 卄 come from combining two 十. But that can't make it "two symbols".)
> But that can't make it "two symbols"
Why not? Same like I can decompose "combining" into "combine" and "-ing" or "decomposing" into "de-", "compose" and "-ing"?