I wonder if Threefish would have been a better option. It's often overlooked.

Threefish has native support for disk encryption, since it has a tweak built right in. So rather than having to use a complex construction like Adiantum, you can just use a single primitive. Threefish is ARX, just like ChaCha. And its speed is within the ballpark (Wikipedia says 3.95cpb for ChaCha20; 6.1 cpb for Threefish).

Whether or not Adiantum as a construction is proved secure, its complexity will lead to increased risk of implementation failure.

In other words, you can choose to attempt to maintain secure implementions of ChaCha, Poly1305, and AES (which is notoriously difficult to get right without side channels). Or ... just implement Threefish (which is as simple to implement as ChaCha).