I feel like the purpose of the coop matchmaking as of now is just to throw you together with people of similar purpose (exploration) (main quest) etc...
Which feels weird because no one likes to deviate from what they are doing or plan to do to suddenly join someone mid way. I suggest to instead do something similar to monster hunter; [Looking for help] [looking to help], have the option to enable public help (showing which 3 quest you picked) and let other players decide who to help (or filter by quests they have too). If that is too complicated maybe just have a SOS Flare to ask people for help.