Right now waveforms have a resolution of 2 points per frame and it's hard to make frame-perfect synchronization between music and video with this, as we don't see the peaks precisely. I even think the waveform is smoothed out because I assume it uses the average amplitude around each point.

Is there a way to display a higher-resolution waveform (or "audio thumbnail" as Kdenlive calls it) ?

maybe can be added a thumbnail quality menu' (liek the one present on video tracks for the thumbails) .... Simple (for fast drawing... maybe the default) ... Medium (like now).. Professional (maximum quality)
also can be requested here: https://invent.kde.org/kde/kdenlive/issues

maybe with a mockup image of the final appearance.

Sure, here it is:

https://invent.kde.org/kde/kdenlive/-/issues/675