Don't you love all the variables! Even in the hamlet I do most of my gardening, heavy clay is predominant on one side of the road compared to the deep silt loam on the other!

When you buy compost you can test it for persistent herbicides by growing some runner beans in it. A lot of the pyralid group of herbicides will stunt growth but won't kill crops but beans are more sensitive to it and cheaper than any lab test. So perhaps give that a go and rule out that as an issue.

Either way perhaps hit it with some effective microbes and see if that improves you situation, it's definitely made a big difference to everything round here.

Wishing you future Dahlia success -atleast they haven't been eaten by deer like they have here- the bastards 😆