Hi @jez
I think what you are looking for is to be able to sort posts using the custom field name as it would translate to sorting by the first letter.
Here is an article that may help you with that.
https://www.advancedcustomfields.com/resources/orde-posts-by-custom-fields/
In order to group them by first letter, you would have to store them in a multidimensional array then use custom control structures to note the change in the start letter.
I hope this helps.
