All over France, an IFR flight may not descent below 3500ft without the ground in sight, unless on an instrument approach.
I've got the feeling it's not as clear-cut as that. I can well imagine that the en-route phase of your IFR flight has to be conducted at 3500' or above. That's got to do with radar and radio performance - below approximate this level there's all sorts of ifs and buts associated with radio and radar performance, and you don't want to worry about those when flying IFR.
But the exceptions to flying IFR below 3500' cannot be limited to instrument approaches only. For starters, instrument departures have to be flown partly below 3500' as well (duh). But also, near airfields, ATC has to have the ability to vector you onto the instrument approach while your altitude is below 3500'. I mean, you don't intercept the ILS from 4000' when being vectored, right? That's typically done at 2000' or so.
So my gut feeling is that within terminal areas ATC can vector you around at 2000'-ish while fully complying with IFR. The minimum altitude in such areas would be the sector altitude, or something else that's locally established based on geography, radar performance and maybe some other factors. But definitely lower than 3500'.
And if ATC can vector you around in their terminal area at 2000' or thereabouts to set you up for an ILS approach, they can also vector you around in their terminal area to your designated IFR-to-VFR transition spot. And the chances of being in VMC with the surface in sight are significantly higher at 2000' than at 3500'.
Then again, this requires that you make the IFR-to-VFR transition near a major airport, and they might not be happy with the additional workload, if that airport is not your destination. Plus, that airport may not be conveniently located next to your destination. But if your IFR-to-VFR transition is planned to take place far away from a major airport, then the minimum at which this can happen may indeed be 3500' or even higher.
But regardless of the actual altitude at which the IFR-to-VFR transition has to happen, it is something to think about and plan ahead. Because there will be a minimum altitude at which ATC can control you while IFR. And you have to find VMC conditions at that altitude, or else you cannot make the IFR-to-VFR transition.