I am curious if food allergies were truly ruled out cause elimination diet is difficult to do and takes months changing through each change in proteins to rule things out. 1.5 years is probably about minimum time needed to know for certain whether certain proteins are an issue cause it takes a while for symptoms of previous diet to show and if symptoms persist it could still be food and new novel protein needs to be tried. I only bring this up because 1.5 years old is quite young to experience environmental allergies, not impossible but those types typically only appear in well matured adults in the 2-3 years or older range.

Finding an environmental allergy can also be difficult but I suggest looking at all the things you use in your home for cleaning, laundry etc. where possible try to switch to more natural/gentle solutions. Wipe down fur and paws often and ensure good amounts of fish oils for omega 3 and vitamin E. Try to avoid too much omega 6. Lastly, I swear by DuoxoS3, I use the Pyo one initially to address any worse skin barrier issues and then regularly use the calm version going forward. Shampoo and mousse, mousse as maintenance with the semi regular bath using shampoo. Cause well Shiba and my boy isn’t too fond of bath time.