Thanks for your response, it helps clarify my thoughts a bit more.
Regarding Nauticam prices, I found a housing for the Canon R5 II on the same dealer’s website:
fisheye-jp at ¥598,000 (excluding the 10% tax), which converts to about €3,773.
Same for china, detail.tmall.com, which converts to about €3,523.
In comparison, the same housing in France costs approximately €4,700 : photo-denfert .
I haven't checked prices from other brands yet.