From what I know, it's a mix of Damus, Nostter.app , Rabbit & Amethyst. Old Iris also gets a lot of love 💜
New users tend to be from Damus/Nostter mostly.
(Not sure if Primal, Nostur, Snort, Coracle, Nostrudel is used much 🤷‍♀️) 

 Is there a relay that I need to add to get the full experience? 

 Yes. yabu.me read-only relay but I think you already have it 
(It's geo-restricted for writes. So for non-Japanese users it's read-only)