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That sounds like it would involve a lot of digging and a lot of building - and I assumed the wall was built on top of some kind of foundations. Could the floorboards not be laid on top of that somehow?
tbc-
You need to clear of the debris under what was the wall and hearth to assess.
Could be that you can reduce the height of the hard standing and batten onto it at the same height as the joists holding up the wood floor. This will then support your new floorboards.
Whatever happens you will have a height difference between the kitchen and the floor that will need some kind of reducer bar.
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It's a bit hard to see exactly what is there in the pictures but it looks like you have a hearth still in place, which is level with the floorboards?
I don't see how you can avoid breaking that out if you want to keep the same levels.
Bobbo
ChasnotRobert
Extend joists, add brick piers to support as required?