From the scrapping of the belt category, we know that it is possible to take armoury chest slots from one category and tack them onto a different category.

It seems to me that the offhand tool category would be excellent target for reassigning a fair amount of its space to other categories.

At its tightest (and assuming it would stay in rows of 5), you could probably get by with only 15 blocks in the offhand slot, and distribute the other 20 plus the 5 kept in reserve from the belts to give each of the left-side categories an extra row.

If 15 is too tight... I'm not sure what the alternative would be. But you don't tend to amass offhand items in the same way as other categories, since tools don't drop from dungeons and there's no point to keeping old ones after you replace them. Any amount of fiddliness it would add to managing your offhand slots would probably be well made up by less fiddling with the other categories.