I was going to say I miss nothing but you reminded me of what I really miss. Mac Preview. It was so versatile and did a lot for a little built in program.
I used to use Sushi for gnome but it never did all file types and it stopped working for me a while back. I have never gotten it to work right again since.
I really don’t mind the single spam I’ve gotten so far and I accepted the request just because I enjoy playing with them.
SimpleX seems pretty nice but I had enough trouble converting my family to Signal. Did you there isn’t feature parity between the iOS and Android versions?