[Affinity Designer: measure line/path length]

+1

Also would like to be able to use Designer for patternmaking work, and therefore being able to know the length of paths is absolutely crucial.

Considering that users have been asking for this for over 6 years, with little sign of interest from the developers, it is not realistic to wait, and more prudent for us to look for alternative apps. For those of you in the same situation, you may want to consider Inkscape (https://inkscape.org). Although it is not nearly as polished and refined as Designer, it does however have a built-in, easy-to-use 'measurements' extension that allows you to work with path lengths (and areas, and more) - and it's free.