Make a new setting that when adding hotbar slots, you can choose between the current and default system that use one row on your current inventory, or a new system that let you modify the number of inventory slot of the hotbar, separately from the slots from the rest of the inventory.

Here is a mockup i made as a concept to compare the current system and the new one, both using 18 slots for the hotbar:

And with the option that lets you change the number of hotbar slot, you can add more inventory rows in your hotbar (up to 3 hotbar rows), or reduce it to 1 slot, and separately of the rest of the inventory that can be already modified on its own.

Also, currently, when you reduce the number of Hotbar slots, those "removed" slots still exists when you open your inventory (as seen in the following screenshot from the video presentation), with this new suggested setting, removed hotbar slots would actually be removed from your inventory.