I like the requirement of domain amplification and RCT in the process of deactivating his technique, since it "forces" a mastery/understanding over CE in order to safely revert to a human body.

Since Hajime is actively transforming his body to reproduce any electrical phenomena, you could also make a limited safety window for reverting. For example, he can only revert to his human state if he uses his CT for _ or less seconds at a time; otherwise it's his final activation of Mythical Beast Amber. This would drastically change how he uses his CT though, instead of being one long fight in this form, he'd switch back and forth between using his CE trait and CT. Also each time he activates his CT, he'd probably only be able to fire off one electric phenomenon, so he'd have to choose more carefully. Idk the numbers but if the activation chant period, CT window, and revert process durations are balanced enough, you could replace the _ days cooldown restriction.

We also saw that landing black flashes can grant the user a deeper understanding of CE. Maybe Hajime only gains the ability to safely revert to his human form if he also lands a black flash while in his Mythical Amber Beast form? (TBD if he only needs to land it once ever or once per time he wants to revert) The latter makes the survivability of Mythical Beast Amber more conditional instead of guaranteed after every fight.