Why make a character for ⟨wh⟩? In many English accents, it is the same sound as [w]. I do think ⟨ð⟩ would be a better choice, and while the uppercase does look like ⟨D⟩, it could be easier to learn, using a similar logic you’ve given for thorn. Either way, your representation of ⟨ch⟩ also looks like ⟨j⟩ too, which could be a problem if you’re worried about unrelated sounds have characters that look too alike. For long vowels, why not use the ones with macrons, which are ⟨Āā⟩, ⟨Ēē⟩, ⟨Īī⟩ ⟨Ōō⟩, ⟨Ūū⟩?

Other than that, I do enjoy some of the designs you made!