I hope your Megane does not suffer the same issue as the Honda I have. In my car, the head unit is the weakest point in the system- just swapping out the head unit makes a bigger difference than swapping out the speakers and powering the new speakers from said oem head unit.
It's a real ball-ache though to do that and you already said you don't want to, which leads me to comment I wish to add. I was lucky enough to find a solution to enable me to swap out my head unit. But if I didn't, and I knew what I know now back when I bought the stuff for my car, I would have looked at something like this (there may be other brands that are better, or more preferred, if you have a favourite)-
AudioControl D-4.800. I'm not familiar with this brand myself, but I stumbled upon it while watching quite a lot of videos on the CarAudioFabrication channel on youtube- and the handiwork of the guy almost made me wish I did what he did for a living. I think though that the stuff he uses is fairly decent- might even be great, but I'm not an expert.
I'm mentioning this because I'm not sure how good a source the Megane oem head unit is- and if it's crappy like mine was, then aftermarket amps, speakers and subs will only get you so far- garbage in, garbage out, and all that.
So I would suggest at least looking at a similar product to get the best out of any new equipment you add to the car.