Google ‘detolf adjustable shelves’ to customise the placement of your shelves.

If space is a little too squeezy, perhaps cycle 1 titan and some smaller figures out of the cabinet to limit their dust accumulation.

Even if the answer is to buy more glass, consider rearranging your collection as the smaller figures can fit in anything. For example Kmart homewares may have perspex boxes, terrariums, jewelry boxes etc which can look good and cost far less than another big cabinet.