One dry and crumbly stool one day, one soft and mucussy stool the other day

I adopted an adult Kelpie Cross from the Shelter about a year ago and never had problems until 2 months ago.

My dog has been on LifeWise ever since I adopted him and we tried different flavours and he was good on all of them. I always used to sprinkle probiotics in his diet. Then I wanted to switch up my dog's diet and got him some Prime100 slow cooked salmon wet food, but he immediately started having very soft stools and started defecating and peeing on it inside which he has never done prior to that. I stopped giving it to him but he kept having these very smelly soft mucus stools inside when we were not home. I first was worried that he had those stools because he developed separation anxiety and then realised I introduced him to a new flavour (Tuna) of Lifewise and he might have a sensitivity for fish. So I stopped giving that as well and started him on a flavour that he was previously good on and added some ZiwiPeak wet food as a meal topper. Again, he kept pooping in the house almost every day when we weren't home. I then thought that maybe the ZiwiPeak was the issue, so I stopped giving that and he is currently just on his Lifewise Turkey and Lamb. For some reason he started pooping very dry and crumbly poos on the one day and the other day back to mucus soft poos or sometimes even diarrhea. I have ordered the probiotics again to try if that makes it stop.

Today I have read that lack of exercise and not cleaning food and drink bowls enough can cause G.I. issues and now I am worried that I caused the whole issue by not taking him out enough (I am not, I am working on it and that definitely motivated me to take him on a walk at least once a day) and not cleaning his bowls enough (I did not think it was that important but I was shocked by reading up on it and embarrassed that I haven't been cleaning his bowls enough). But would a combination of those things cause the whole soft poop one day, crumbly poop the other day issue? And the pooping inside, or is that a whole other issue?

It is probably worth mentioning that otherwise he is a healthy dog (except for some skin issues) who has the perfect statue/weight and never vomits, is not lethargic and is very happy in general.

I will go to the vet tomorrow but before making assumptions I want to know if that makes sense?