Flat roof repair and insulation advice

Hi, need some advice to repair and insulate a flat dormer roof please (see pic).

Current roof is felt and has been leaking for some years. There is no insulation in the roof void in at least one of the rooms. There may be insulation in the other room as this was added later, and does look to be slightly higher.

Would it be best to keep as a cold deck and add celotex insulation in each room, and replace the felt with EPDM or Fiberglass?

Or would it be better to replace the whole thing with a warm deck? Would I need to remove any existing insulation in the roof if I did this? Would it cause problems for my neighbour with my roof now being higher?

Any recommendations on EPDM vs Fiberglass?

Thanks.