Yes, not being able to divine how this will go on any given flight is challenging and maddening. No easy answers here. I have flown AF business class a few times since 2013 (most recently May/June 2023), and never had a problem with carry-on bags (I also check my larger bag). I use a soft Briggs & Riley carry-on (small duffle bag, no wheels), which easily fits in the overhead, with room to spare for another carry-on. I don't know the size of that bag, but it is certainly smaller than 21".
In general, you can expect a more generous allowance for carry-on in business class as the overhead bins will be larger than those in economy (with fewer passengers using those bins). I suspect that your PHX-CDG flight may not be a problem if your carry-on is 21 or 22 inches (unless they indicate that's not allowed now). However, your CDG-ATH plane will be smaller, so you may have smaller overhead bins. BTW, I have had fairly good experiences when calling Air France. You could inquire about this matter, and ask specifically whether there will be different space parameters for each of these flights.
I'm surprised by Lony's experience (being asked to check bags despite empty overhead space), although Lony did not indicate if this was a business class seat. That is quite annoying all around, no matter what kind of seat you paid for!
Apologies for stating the obvious, but no matter what size carry-on you decide to bring, it's a good idea to board the plane as soon as you're able (they will board biz class first). In theory, with early boarding you should find empty overhead bins ready for your stuff. That's one of the business class perks. If you board later, it's possible that other passengers who struggle with the very limited overhead space in economy will need to store their stuff in the business class overhead bins. I've seen it happen. You know how it goes: at some point the flight attendants just want everyone in their seats and bags stored so the flight can depart on time.
And speaking of perks, you can expect some very nice meals, particularly on your AF transatlantic flights. Excellent wine selection too. And then there's the cheese course...