Goldmaster, iirc f-droid builds apps on its own CI server and signs them with its own key. Also google now requires app bundles because of course it does, "size-optimizing an apk for a particular device" is of course very important right when everyone is done ditching bitmap graphics (sarcasm). And I don't have a good idea of how signing is handled in that case. So no, having the same key on both stores probably won't work.