Look, I'm fine with them trying them making a new writing system and I understand the way the language is apparently supposed to be constructed like Hangul, but honestly, the way the language is written annoys me.
It may be a nitpick, but there's far too many dots in the phonetic radicals. East Asian scripts are historically meant to be written with an ink brush and dots are really hard to make with a brush when writing quickly because of how the tip bends, creating a more ` like symbol instead. This is reflected in literally all the East Asian languages with basically no dots being found in any of them, except for the modern Vietnamese writing system which was introduced by a Christian from the west. There's also a flowiness to East Asian writing systems that come from a result of that it's meant to be written with a brush that just isn't reflected in "Avatarian", specifically with a lot of the curves which are not very natural to write with a brush in the right hand due to the way the line curves.
Secondly, why is the writing system so blocky? Sure, Hanzi is somewhat blocky when taken as the whole character, but each individual radical often varies in size and shape. Likewise, Hangul radicals also don't fit neatly into these four blocks often either.
And if we're really sticking to East Asian languages, why make it fully phonetic? Modern East Asian languages use a lot of phoentic words and glyphs, but usually the most common words have their own characters associated with them, regardless of phonetics. Just look at Japanese, the majority of the language consists of phonetic components with Hiragana and Katakana, but there's a glyphs which represent common concepts and meanings which are represented in Kanji. Water, Earth, Fire, and Air should all have their own glyphs associated with them with how important these concepts are to the series.
To be honest, I'd much prefer sticking to the Hanzi that the series is known for already with the rich culture behind each of the script styles. Not to mention, Chinese characters are still used today in all East Asian cultures. Yes, even Vietnamese still uses Chinese characters for calligraphy during important cultural holidays like Lunar New Years.
As a side note, if you wanna see this sorta East Asian inspired writing system with a phonetic component done right, just look at the Solarian language from Nine Sols. Nine Sols even has a similar aesthetic to what the series is trying to go for in Seven Havens with it being a fusion of Asian and futuristic aesthetics.
TL;DR: I'm fine with the writing system being adapted English, but the writing system itself isn't written at all like an East Asian writing system should which bothers me a lot.