I host my own instance with Masto.host. (@[email protected])
I don’t need to be concerned with the Threads drama or worry about people defederating it - I’m in control.
As for best client, I personally love Mona. It runs on my iPhone, iPad, and Mac and perfectly syncs my position in the timeline across devices. Lots of other great features, too.
iPhone 6 updates ended with iOS 15. Still, that’s a pretty good run. Unfortunately, you can’t root an iPhone and install a custom ROM like you can with Android.