I can really relate to you with Darwin - Ollie however is with the bladder ( he is 13 and on steroids and water tabs due to illness)
Like many cockers he has always had sensitive tum and it flares up more now. While I manage it well with a a mix of 3 things Natures Deli wet food either salmon or senior, with added white flaked fish and and senior chicken rice kibble - I did discuss his tummy a couple months back with vet and she suggested I look to Hills I/D which I understand is in kibble and wet tin food (Ollie would not eat kibble alone
) while I have never used it I was told the output on Hills would be less. So it will be my back up if needed. The output was because I mentioned he has bouts of the runs.
For his flare ups I use Dorwest bark powder which is slipper elm it usually settles the runs down within a day or two and I just give white fish and boiled rice. The other suggestion would be probiotics which help the digestion, but I think with Darwin it is different and muscle nerve loss.
I hope others come along with advice soon.
Take care