Repotting in flower IS good- I do it during week 3, when stretch is over, and I go through and look at my plants and see who looks like she's going to finish too big for her pot.
I don't know anything about the bark component of potting soils- if there's bark, it should be well composted and pretty well ground up. Are you sure that's not a bagged mix being packaged as 'top soil'?
Honestly if you suspect your soil sucks, instead of trying to make it not suck (which more than likely won't work or be more trouble than it's worth), why don't you run out and grab a bag of pro-mix potting soil or even just regular potting soil (no extra nutes though!). That will get you through to harvest anyway and you can go back to mixing your own from better ingredients next round if you like DIY media, or go shopping for a new type of bagged mix.