This is not the groomers fault, and like someone else mentioned it is a result of miscommunication
As a groomer, I personally know that we are educated on grooming certain breeds. However there is no breed standard cut for cavaliers like there is for say a schnauzer or a cocker spaniel.
So this excessive trimming is just a miscommunication between the two of you. You MUST be clear about exactly what you want, otherwise you won't get that
They will do what they prefer if you don't specify what it is that you want, I have had to do this many times before for all types of dogs because the owners aren't specific enough.
The correct grooming for a show cavalier is no trimming at all, or minimal unnoticeable trimming, however most pet owners do want trimming.
Or even full clips, I have many cavalier clients that want their dog shaved except for the tail and ears, i have some with minimal trimming, some who want no feathers, some who want the ears bobbed, some who want the feet shaved etc. So the groomers are educated to do all of the above
but as you can see none of them are all the same, it depends on the owners preference. So be crystal clear about it, we do appreciate owners who take the time explaining exactly what they want. And being clear about it isn't the same as being picky, so don't worry about that
I am doubting that your library will carry "notes", so I will message you the information from it for you.
The drawing on a picture with red pen is a very good idea, so is taking the dog, putting it on their desk etc. and showing the groomer exactly how much you want trimmed off each area.
Even taking in a photo, or writing it down as well as showing them would be beneficial
And in the end, just remember it will grow back
I am sure he is beautiful even with short feathers