Boom. Have you forgotten about me yet?
This "power kit" solution is not satisfactory, and does not help.
I had around 100-ish legacy mythical items left (that I'm willing to get rid of)... I turned them into power kits... and it turned into 157 (!!) power kits... so I got a net item gain.
This does NOT help in getting under the item limit for the grace period at all, since the fusion costs to use these power kits are the same.
At the very least, make it so that power kits (and color kits for that matter), do not take up any inventory slots. This will also give a purpose to fusing items into non-maxed power kits, just to lower their item count for the item limit.
...or you know... remove fusion costs.