Insulating a house with a flat roof?
TL;DR: Does anyone have any experience with or advice about insulating the outside walls and roof of a house (especially one with a flat roof)?
We moved into our house in 2016 and after being super cold our first winter, we upgraded our HVAC to a ductless electric heat pump system. The new system works great but we have to run it more or less constantly to keep the house warm, which is astronomically expensive. The house has a stucco exterior and a flat roof, and we've been told that there's no insulation anywhere. I want to put in solar but it seems clear to me that the first step should be to insulate the house, which will likely involve replacing the roof.
As a first-time homeowner, I'm leery of charging into this kind of major project and not sure of how to proceed. Has anyone done this kind of thing before? Did you use a general contractor or hire individual contractors? (It seems like a GC would make sense, considering there are likely to be roofers, insulation pros, and eventually solar pros in the mix.) Should we be thinking about the solar as part of the roofing operation, or is the insulation project totally separate? Any recommendations or stories about projects like this that went right or wrong would be very welcome. Thanks!