Veterinarians or vet techs!!!

Hi everyone, so my dog is a pure bred golden that is 6 years old. Around may 30th he had bad colitis which was treated with four types of medicine (his poop was diarrhea but then switched to mush). Right after, he got hot spots, and was put on medicine immediately following the end days to the colitis medicine. Ever since, he has had soft stool. Not diarrhea, but not firm either. I took him back to the vet to make sure no unwanted bacteria regrew or if he got colitis all over again. However the vet assured that nothing was in his poop and was more of a diet issue. I did exactly what she said and switched him to royal canin, and yet his poop has still been consistently soft. It’s manageable to pick up, yet always leaves some residue. What can I do? Is this just his new poop? I am very protective of my dog and I have crippling anxiety thinking something is wrong again and I’m overlooking. But I don’t want to spend another $200 for the vet to say it’s a diet issue once again. (Money isn’t the issue, but I don’t want to waste it on a pointless trip for her to say it’s another diet problem)