Depends on what kind of concise we're talking. Those are in the BMP, so two bytes a piece. The keys are fairly short though, 2-3 bytes is pretty compact for useful keys in most JSON.

The placed number system without Arabic numerals is weird to me. I get that it's probably not all that efficient decoding long-form Chinese myriadic numbers, but it's still weird to see 「九〇〇」instead of 九百 and 「六一」instead of 六十一; though maybe I'm the weird one for thinking that looks weird.

Do you prefer 丗 than 三十/三〇

That would be a bit more obscure, but very classical!