I did a research few days ago about this. My findings; step 1) remove the old plant and the bigger leftover roots from the soil step 2) flush it again to remove old nutrients (something like cannaflush can help with it as a boost) step 3) add enzymes to break up leftover roots and turn them into nutrients (something like cannazym or sensizym) step 4) add worm casting step 5) keep it moist so the chemical process can do it's thing step 6) let it rest for 2-3 months. Make sure the soil can breath and the chemical process in the soil can proceed in each part so every 3-4 days mix it with a shovel. I'm a newbie so if I missed something, experienced growers may comment on this. I want to learn. I bet the PH value of the soil needs to be around 6.2?

No, no no! Use fresh soil each and every grow, there will be no nutrients left in the old soil and the risk of diseases is far too great. Soil is usually the cheapest part of any grow, trying to save money here will lead to poor results and a lot of wasted time. You only get out of it, what you put into it! Cheap dirt = cheap, low quality buds.

Please use fresh soil every time. in the old one there may still be root remains, small cattle or other things inside. Only the fertilizer for the start is definitely no longer inside. So please don't skimp on the few bugs

you shouldn't, since the soil will be without nutrients, but you can fertilize it again by adding worm humus, for example.