Edit: I read that you had problem with head bolt threads, sorry. But some of it could be applied to spark plug threads. Heat transfer from the plug is quite important, a threaded insert always have a parting line that is bad for the heat transfer, so it is better if the threaded insert is silver soldered in an iron head and swaged or crimped in place in a alum. head.

Threads for the cylinder heads in the cylinder is quite short and for that reason the helicoils tend to creep up, and pose a risk for the head gaskets to start to leak. There is a version of thread inserts that is pinned with 2 or 4 small pins half-and-half in the threads that maybe be better in some regards, but the casting is quite soft, so there is always that risk of overtightening the bolt and stretch the cylinder material again.

There is the option to make custom made threaded plugs that is spot welded (or silver soldered) to keep them firmly in place. With that and with modern high strength bolts you got the option to use slightly smaller bolt diameter for the head bolts.

Pin bolts does strain the cylinder material less as thread wear, and force of the twist and shear is transfered more to the top of the pin, and nut.

Cylinders should be honed after that kind of a job, preferebly with a torque plate.