Think there is more at stake here than language. What does language represent to you?
Clearly it is very tied up with identity. But honestly, when you settled down with a swedish man in sweden, it was clear you children would not be 'just' or even primarily french. It is important, for example, that they speak the language of their peers in the country they live in.
But my first thought was: it's not called mother tounge for nothing. If you speak to your children solely in french they will speak it back to you. But once they get to 2/3/4 what happens out of the home matters.
examples:
Im a brit married to a french man. We live in the UK. DC are billingual, although spoke english first. been in billingual french / english nurseries since 1 yr old, but french only became fluent around 3-4 (english was fluent an advanced 18 months earlier). Mother toungue english is reinforced by living in UK. Without billingual education i doubt they would have fluent french. DH tries to be consistent about it, and they'd understand and speak it, but billingual education when young supports it massively. For the teenage year i think other things are more important in education choices and DC can maintain their french other ways.
BIL and SIL also live in UK. French / german couple. My nieces are trilingual, as your children are likely to be. it seems to me that
- their first language is english (they are teenagers) don't know what it was like when they were little but think the drivers here are that the parents speak to each other in English so it is the family language and they have done 100% english speaking at school
- second language german - mother tongue, they still speak it with their mother
- third language french - their dad has been hands off and honestly hasn't put in the effort. They speak it well, but not total fluency and i gather written french is poor.
Honestly, as an english speaker their english isnt native speaker perfect either. But on the other hand they are trilingual, which is a huge skill. But as languages get added, there is a trade off with depth.
For you: if you want french to predominate, you either need to go for french / billigual eduation, or speak it as a family at home.
WHy are you sad about it? Can you turn it into a positive? To be bi or tri lingual is a massive skill.
Also, in a mixed family, it's important they speak both to have relationships with the wider family. I'm supportive of DC's french, so they can know grandparents, cousins etc. You signed up for a mixed family, so you both need to support that. Im supportive of billingual education here. If we moved to france, i'd want billingual or english schooling.
Once the baby is here, I think you will have bigger issues than language to worry about. There is a big variation in cultural norms on how to bring up children. Broadly i see a split between northern europe (scandis, germany, netherlands, UK) which is more relaxed and needs of the child led, and southern europe (france, italy, spain) which is more discipline and control based.
Honestly to me the standard french approach to parenting seems about 40 years behind. We have regularly family disputes (from DH's parents) e.g. on whether to use physcial discipline which i very normal in france but very discredited in the UK.