Thank you very much for all your replies and interesting information.
@bosswrench : I ALREADY replaced the seals and the pistons, and addtionnally lightly sanded the bores, fitted everything in place after smearing some brake grease... and before putting the calipers back, I checked that all pistons moved freely. That's why I was quite surprised AND disappointed !
I checked the tires, yes. There was also no evidence of any grease nor fluid leak, nor pinched or damaged flexible hose, otherwise I would have solved this kind of problem in first place : so, I'm in a fix :-€
You are right, a flexible hose could be damaged without external sign, so maybe I should replace the left and right ones. Except them, I can't see what I could replace in the braking system ! A step forward would be to replace the calipers (maybe one is distorted by the building heat ?), but it has a cost...
As a - new - start, I think I'm going to have the car partially tested on a DOT/MOT station's rolling bench, in order to measure the difference between left and right drag force. Even if I'm 99% sure that this is a brake-related issue, it coud help to exclude any front suspension issue (kinematics, wheel plane movement under effort and so on).
Matt