Yeah...I was missing pears and peaches the whole time, found it so annoying as I didn't unlock them until I got the NSO trial. I went to some treasure islands to get them (although it took ages to even get to one lol) and someone else kindly helped out too. I wish Nintendo had made it so it was possible to get all the fruits without needing to invite/visit people on New Horizons.
I tried this multiple times with the postcards for months and even sent different gifts, no gifts, longer and shorter notes...didn't get the other fruits. I think it's in the game's mechanics that you only unlock native fruit, coconuts and two non-native fruits without other people's assistance for the other two non-native fruits you need. (Correct me if i'm mistaken though, there's still alot I don't know about New Horizons despite that it's been out two years lol) I used to really enjoy sending letters and getting fruit from villagers in New Leaf!