Once the royal family is solidly Orthodox, they're free to follow the Russian model of marrying women who aren't Orthodox but (at least for the future Queen) are willing to convert (see OTL Greek Queens like Sophie of Prussia, Frederika of Hanover, and Anne Marie of Denmark).
One reason for that is because the notable Orthodox options are rather thin, basically consisting of Russia and (if it still happens ITTL) Romania. As such, the Crown Prince will likely be allowed to marry a non-Orthodox Princess willing to convert, while his brothers will get a bit more leeway by saying conversion would be preferred, but not mandatory as long as the kids are raised Orthodox.
The only issue with a French match for the Crown Prince is that getting Catholic princesses to be able to convert was notoriously difficult, as shown IOTL with Helene of Orleans having her potential marriage to Prince Albert Victor thwarted by her father and the Pope refusing to let her convert until she turned 25 (at which point she would have been an old maid on the marriage market). So unless you get a dad and Pope willing to let the girl become Orthodox, getting a Catholic bride for the heir will be difficult.
However, a Catholic bride for a younger brother is doable, as seen IOTL, as brides not destined to be Crown Princesses/Queens usually had the ability to privately keep their original faith as long as they promised to raise the kids Orthodox