I found Xapros SMP and it is a chill vanilla survival server for adults only. It has players from lots of places so it feels like someone is online at almost any time.
The world is meant to be long term since it started in January 2025 and they say they will not reset. It is a good place for huge projects and megabuilds. Peak time looks busy with around 20 to 25 players online.
It runs on Fabric so redstone and farms act like singleplayer. They also added BlazeandCave advancements if you want extra goals.
There are no land claims but griefing and theft are not allowed. There is a tool to inspect blocks if something goes missing. Travel is easier with a Nether ice boat highway and there is a nice spawn plus a diamond shopping area. There is also no world border so you can live super far away if you want.
