I found this server called The Tavern and it is a fresh vanilla SMP that started about a week ago. It already has a bunch of players and it can hit over twenty people online and still stay smooth with basically no lag.

The server is fully vanilla so it feels like normal Minecraft but with a lot of people around. They keep it up all the time so you can hop on whenever. There is a live map website you can look at and also a stats site so you can see things like how many blocks you placed.

They let redstone builds happen and they also allow things like litematica and minimaps. There are no server resets and there is no world border so you can just keep going and make big bases and farms without worrying it will all get wiped.

The community is really friendly and they do community projects and events. There is a Christmas tree building contest starting soon which sounds fun. It has a Hermitcraft style vibe with people working together but still doing their own thing. There is proximity voice chat so you can talk when you get close to other players in game which makes it feel more alive.

If you want a chill vanilla SMP with events and long term worlds this server fits that really well.