The French health system seems complex, and arbitrary at times, but is quite fair.
Every medical act has a price attached to it, and a percentage that will be reimbursed by social security (or dealt with automatically without you paying upfront).
The percentage varies from 0% - 100%. So it very much depends on what sort of thing you are seeking medical care for as to whether you end up paying very little, or rather a lot.
My drug bill is €1200 a month. Were I not on an ALD I would be having to pay 30% of that, ie €360 each month. So a mutuelle that costs €120 a month would be a great deal!
If you are broadly healthy, and rarely visit the doctors, and don’t have glasses or bad teeth, then you are not going to get much back from a mutuelle.
However we have hospital cover because that is where the bigs bills can stack up. The daily fee is €20, but that’s just the bed. As you have probably realised everything here is itemised so you pay for every bandage, injection, etc. And many hospital surgeons and anaesthetists especially in clinics are secteur 2, so they can charge more than the social security amount.
I looked into the costs of an aortic valve replacement, which is what I might have to have one day. And it was around €25,000 going to a proper surgeon. So having to pay €8-10,000 is no joke.
A knee replacement is about €3000, so the part to be covered would be a more reasonable €1,000. But there would also be about €20 a time for physio, and one can need 20 sessions…
However to go back to the original point, no matter what you will be entitled to you carte vitale. But with potential pensions from 3 countries that could be complicated working out which country will be your competent state.