Ok, so I somehow wasn't expecting it, but Inkscape seems to mostly fit the bill! It generally ticks all the boxes in my original question.
A few small disadvantages with regards to "MS Surface Pen support" are that:
- it doesn't seem to have support for pen pressure sensitivity; or at least I couldn't find it (please correct me if I'm wrong!)
- navigation with fingers could be better (no panning, must workaround with zooming)
- doesn't seem to auto-detect if I'm using the drawing or erasing end of the MS Surface Pen (they both work the same in the app)
edit: One annoying disadvantage I noticed soon afterwards is that, at least as of the version I tried, the drawing line doesn't start immediately when I start moving the Surface Pen, but only a small moment later. You can see this especially as I try to draw the second fragment of "hair" — the line is non-continuous, although I push the pen where the previous line ended. Also, a similar effect seems to be at work where I struggle to draw the pupil of the eye on your left. Not sure what's the reason and if it's possible to fix this. Haven't tested newer version of Inkscape yet either.