Yes please!
I've run into this exact problem. Right now Affinity Designer can export with pixel dimensions (when "set viewBox" is unchecked) or with height="100%" width="100%" (when "set viewBox" is enabled). I'd like to have a third option, to set the height and width using the actual dimensions of the artboard, in either mm or inches. (Ideally one could choose the unit in the SVG export options, but just using the document units would also be acceptable.)
Otherwise one has to play the "guess what dpi setting the software on the other end uses" game, which is never fun. (And even worse is when you need to open your SVG in multiple programs, which all assume different default dpi settings.)