Inverse taper can be corrected fairly easily if you are planning on allowing the tree to increase in height and amount of foliage. If you are not planning on letting it get bigger, then inverse taper is much more difficult to correct.
Slip potting is a practice used only by "houseplant people" and is a poor technique for bonsai. Trees do not like their roots being disturbed. A classic beginner bonsai mistake is to repot a tree too frequently. One should plan ahead, repot doing all the root pruning and nebari work needed then allow the tree to grow at least 4 to 5 years or more without disturbing the roots. Slip potting causes damage, seldom has the benefit one desires. Plan ahead and do a proper repot with root work. Mature bonsai are often allowed to go as much as 10 years between repotting. There are exceptions, sometimes deciduous trees are repotted to deliberately slow down growth. But if you are still working on increasing trunk diameter then you do not want to repot more than absolutely necessary.
I would recommend against going back to an organic mix. Especially if you plan on returning to an inorganic mix as the tree approaches maturity.