Your boys are similar ages to mine (5 and 3).
We don't use opol, as we're both bilingual and think the boys should know that people can be bilingual and learn to swap between languages. We live in the UK, so their spoken French is a bit behind, but they understand everything and watch TV and have stories in French, in about the last 3 months they've both started speaking a lot more French (really decent sentences). No speech delay at all in English.
I don't think the French teach using phonics, certainly their cousins aren't learning that way. So we've been reading French books and just pointing at frequent words, and my eldest is starting to recognise them. At this age they'd be doing very little reading in France, they start much later.
We are planning to move to France for good when my eldest goes into year 2 and youngest year 1, I'm not worried about them being behind. Based on relatives experience when they moved back from America, the French system is entirely unsupportive, but the kids weren't behind and coped fine.
As long as you keep reading and writing in English with them they won't lose that. They should be fine to rejoin.
Since my else's was 3 we've been using age appropriate cahier de vacances, so that they are exposed to school type vocabulary. I'd recommend those. Especially as handwriting counts for so much in France still.
I think you can delay reception before you start, but I think they'd be very reluctant to put a bilingual speaker with school experience in with reception, and I think your ds would hate it. Better to go back into year 1.
I don't think other subjects should be a problem.
Do think about the effects it could have on your oldest a friendship groups and how settled he is. You'll be taking him away from friends in England and then in France.