That just looks horrible and clunky... I understand what it is that you're attempting to achieve (that is to say, preventing transcription errors when writing/entering private keys)... but this system is NOT, in my opinion, a "better user experience".
Aside from attempting to write words in mixed case... you're also added in extra "checksum" words. So, now my private key of "L1zK6x6HnrT9kjcuqpQf1WQZHYBhucBEuZ5qZdPrFkCgJQ6eh6EN"
becomes something like:
LIMA one zulu KILO seven
six xray six HOTEL GOLF
november romeo TANGO nine DELTA
kilo juliet charlie uniform delta
quebec papa QUEBEC foxtrot sierra
one WHISKEY QUEBEC ZULU five
HOTEL YANKEE BRAVO hotel ALPHA
uniform charlie BRAVO ECHO XRAY
uniform ZULU five quebec nine
ZULU delta PAPA romeo MIKE
FOXTROT kilo CHARLIE GOLF papa
JULIET QUEBEC six echo one
hotel six ECHO NOVMBER TANGO
I'm very familiar with the NATO phonetic alphabet... I use it all day, every day at work... and typing all that just now was painful AF. Not to mention that it is visually quite jarring to look at.
Personally, I think that a system similar to BIP39 is the way to go. You'll end up with 24 words instead of 65... and it won't matter about case... because I can guarantee that someone will write them down incorrectly and write them as "lima one zulu kilo seven six xray six hotel golf november romeo tango nine delta..." or vice versa. Make something foolproof, they'll just start making better fools!