The best way is to put them into good soil,  then you don’t have to worry about them for a year or so.     I think a shredder is a good asset if you are cutting down a lot of plant.  I do that and then add it back on top of the soil.

I think texture is probably more important than the pH and all that for salvias.  I like to use a mix of about 50-50 mushroom compost and sand for beds that need raising.   There are no weeds in the mushroom compost or the sand.   It is virtually impossible to buy really good soil.  Sometimes they make up a mix with good constituents but it is often fresh.  I am lucky to have a mushroom farm not far away.   I get a truck load and age it for a year or two.