A symbol is highlighted with a green frame when hovering over it and with the same green frame when it is selected. I would like a distinguishable visual indicator for when a symbol is actually selected.

Maybe just finally introduce resizing symbols, then you can display handles around the selection frame, just like any other object! ;)
-> https://adobexd.uservoice.com/forums/353007-adobe-xd-feature-requests/suggestions/16939339-scale-resize-symbols-individually