Levelling hearth - what to use?
It sounds like a Fireplace Screen or Cover and not an "Insert" with its own firebox. But, the fireplace floor must be replaced with Firebrick using High Temperature Fireplace Cement/Mortar and according to your local regulations, may need to be slightly lower than the hearth.

The hearth Slate that you plan, should be laid and leveled on a bed of freshly poured fireplace cement/mortar (not the old stuff) and, by regulation, may need to be pitched back toward the fireplace so stuff can't roll onto the combustible floor...regardless of The Insert.

Due to the re-purposing of an open fireplace to now contain an Insert you may, by regulation, be required to blanket and surround The Insert with high-heat resistant Mineral Fiber wool or board. Check Your Regulations.