For a plug thread...Timeserts are the way to go.
Plugs are in and out far more often than a lot of fixings, and the area is getting a LOT of heating and cooling.
With the head off, you can get a good look at the other plug threads, and make sure all is well there. Done in situ, I would laways worry some swarf will get left behind in there, and damage the bores when you run the engine. Probably not going to happen.....