We already know that self-blue to self-blue will impact feather quality and self-blue is just another dilute gene. And, eventually, the blue might become so light as to be unrecognizable. If I had enough years left, and the space to do it, it would be very interesting to see the F3 or F4 generation of blue to blue...but I don't have the space and doubt I have that much energy.
Some things we 'know' are not necessarily true..Lavender to lavender may not be the problem...most breeders are using black to improve type by breeding lavender to black splits...feather issues are still there ???...I went way past F3 -F4 breeding blue to blue rocks, and they are very recognizable, with great feather quality
Don't have them, don't want them, so can't argue the point.
I'd be very interested to see pics of your F3-F4 gen of blue to blue rocks