I'm not sure where you are but I noticed comfrey is more sedate in SE QLD than when I lived in Melbourne. It still grows strongly but loves moisture for its big green leaves. My first few plants grew slowly but then I was able to divide and they quickly multiplied. The roots go deep and mine nutrients but they must also exude a lot of goodness because they attract a lot of soil microbiology and make the soil soft. I found that keeping a small patch going in good soil allowed me to create lots of roots to put into poorer soil and start more plants. To transplant I just grab the top and pull like a carrot. It's pretty unlikely that you will remove all the roots and a new plant will soon be up stronger than ever. The roots I cut into 1 - 2 inch pieces and stick an inch underground.
High nutrient levels particularly phosphorus stop plants establishing mycorrhizal fungal relationships. Tilling breaks the mycelium threads that connect the fungal web together and mycorrhizal fungi require living plant roots to create carbohydrates for them (which is why I try not to lay beds fallow). Fungicides can also be an issue (but I'd wager not at your place so much). Fungal spores are everywhere and innoculants are a waste of money (IMO). I bought some mycorrhizal innoculant for $50 and saw good results on tomatoes in neglected soil, but not in good soil. Mushroom compost is good as an amendment but not as an innoculant.
High nutrient levels particularly phosphorus stop plants establishing mycorrhizal fungal relationships. Tilling breaks the mycelium threads that connect the fungal web together and mycorrhizal fungi require living plant roots to create carbohydrates for them (which is why I try not to lay beds fallow). Fungicides can also be an issue (but I'd wager not at your place so much). Fungal spores are everywhere and innoculants are a waste of money (IMO). I bought some mycorrhizal innoculant for $50 and saw good results on tomatoes in neglected soil, but not in good soil. Mushroom compost is good as an amendment but not as an innoculant.