@Zverik and @tyr_asd:

Thank you for tips, very helpful. It’s amazing what can be done with JOSM.

I have a word of caution for the Relation Toolbox though (and yes, it’s about style): it makes it very easy to create lots of multipolygons from line segments (instead of closed lines). This may be the better option in some cases, but also can make an area highly complex for mappers who are not experienced. For the most part, I therfore prefer to work with closed lines.